American Football

American football (known simply as football in the United States, andgridiron some other countries) is a sport played by two teams of eleven players on a rectangular field 120 yards long by 53.3 yards wide with goalposts at each end. The offense attempts to advance an oval ball[2]  down the field by runningwith or passing it. They must advance it at least ten yards in four downs to receive a new set of four downs and continue the drive; if not, they turn over the ball to the opposing team. Points are scored by advancing the ball into the end zone for a touchdown, kicking the ball through the opponent's goalposts for afield goal or by the defense tackling the ball carrier in the offense's end zone for asafety.

American football and its rules evolved from other codes of football, rugby andassociation (soccer). The first game was played on November 6, 1869. A set of rule changes drawn up from 1880 onward by Walter Camp established the snap, eleven-player teams and downs. Later rule changes legalized the forward pass, created the neutral zone and specified the width of the football. It is one of the two main forms of gridiron football, the other being Canadian football.

American football is the most popular sport in the United States and the National Football League's championship game, the Super Bowl, is among the most-watched sporting events in the world.

Contents
[hide]  *1 History
 * 1.1 Early history
 * 1.2 Evolution of the game
 * 1.3 The professional era
 * 2 Etymology and names
 * 3 Teams and positions
 * 3.1 Offensive unit
 * 3.1.1 Backs and backfield
 * 3.1.2 Linemen
 * 3.1.3 Receivers
 * 3.2 Defensive unit
 * 3.2.1 Defensive line
 * 3.2.2 Linebackers
 * 3.2.3 Defensive backfield
 * 3.3 Special teams unit
 * 3.3.1 Field goals and kickoffs
 * 3.3.2 Punts and punt returners
 * 4 Equipment
 * 5 Rules
 * 5.1 Scoring
 * 5.2 Field
 * 5.3 Duration and time stoppages
 * 5.4 Advancing the ball and downs
 * 5.5 Kicking
 * 5.6 Officials and fouls
 * 6 Gameplay
 * 7 Leagues and tournaments
 * 7.1 Minor professional leagues
 * 7.2 International play
 * 8 Variations and related sports
 * 9 See also
 * 10 Footnotes
 * 11 References
 * 12 External links

History
Main article: History of American football===Early history=== American football evolved from the sport of rugby football.[3]  The first football game was played on November 6, 1869 between Rutgersand Princeton. The game was played between two teams of 25 players each, used a round ball, and resembled a combination of rugby and soccer in its rules. The ball could not be picked up or carried, but it could be kicked or batted with the feet, hands, head or sides.[4] [5]

Collegiate play continued for several years in which matches were played using the rules of the host school. Representatives of Yale,Columbia, Princeton and Rutgers met on October 19, 1873 to create a standard set of rules. Teams were set at 20 players each, and fields of 400 by 250 feet were specified. Harvard abstained from the conference, as they favored a rugby-style game that allowed running with the ball.

An 1875 Harvard-Yale game played under rugby-style rules was observed by two impressed Princeton athletes. These players introduced the sport to Princeton, a feat the Professional Football Researchers Association compared to "selling refrigerators toEskimos."[5]  Princeton, Harvard, Yale and Columbia then agreed to intercollegiate play using a form of rugby union rules with a modified scoring system.[6]  These schools formed the Intercollegiate Football Association, although Yale did not join until 1879. Yale playerWalter Camp, now regarded as the "Father of American Football,"[7]  passed rule changes in 1880 that reduced the team size from 15 to 11 players and instituted the snap to replace the chaotic and inconsistent scrum.[6]

Evolution of the game
A photograph of Walter Camp, the "Father of American Football", taken from 1878 when Camp was captain of Yale's football teamThe introduction of the snap resulted in unexpected strategy changes. Previously, the strategy had been to punt if a scrum resulted in bad field position. A group of Princeton players realized that, as the snap was uncontested, they now could hold the ball indefinitely to prevent their opponent from scoring. In 1881, both contestants in a Yale-Princeton game used this strategy to maintain their undefeated records. Each team held the ball, gaining no yardage, for an entire half. This "block game" proved extremely unpopular with spectators and fans.

A rule change was necessary to prevent this, and a reversion to the scrum was considered until Camp passed a rule in 1882 that stated that a team would have three downs, or tackles, to advance the ball five yards. Failure to do so would forfeit control of the ball to the other team. This change made American football a separate sport from rugby, and the resulting five-yard lines added to the field made it resemble a gridiron in appearance. Other major rules changes included a reduction of the field size, to 110 yards long by 53.3 yards wide, and the adoption of a scoring system that awarded four points for a touchdown, two for a safety and a goal following a touchdown, and five for agoal from field. The last major remnant of rugby was removed in 1888, when tackling below the waist was legalized.<sup class="reference" id="cite_ref-Camp_and_His_Followers_6-2" style="line-height:1em;unicode-bidi:-webkit-isolate;">[6]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">Football remained a violent sport despite these innovations. Dangerous mass-formations like theflying wedge resulted in serious injuries or even death.<sup class="reference" id="cite_ref-8" style="line-height:1em;unicode-bidi:-webkit-isolate;">[8]  A 1905 peak of 19 fatalities nationwide resulted in President Theodore Roosevelt's threat to abolish the game unless major changes were made.<sup class="reference" id="cite_ref-9" style="line-height:1em;unicode-bidi:-webkit-isolate;">[9]  Sixty-two schools met in New York City to discuss rule changes on December 28, 1905. These proceedings resulted in the formation of the Intercollegiate Athletic Association of the United States, later named the National Collegiate Athletic Association (NCAA).<sup class="reference" id="cite_ref-NCAA_10-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[10]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">The legal forward pass was introduced in 1906 after its suggestion by John Heisman, although its impact was limited due to the restrictions placed on its use. Further 1906 rules changes included the reduction of the time of play from 70 to 60 minutes and the increase of the distance requirement for a first down to 10 yards over three downs. Additionally, the neutral zone was created along the width of the football.<sup class="reference" id="cite_ref-Blondy_Wallace_and_the_Biggest_Football_Scandal_Ever:_1906_11-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[11]  Field goals were lowered to three points in 1909<sup class="reference" id="cite_ref-NFL1911_12-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[12]  and touchdowns raised to six points in 1912.<sup class="reference" id="cite_ref-NFL1911_12-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[12]  The field was also reduced to 100 yards long, but two 10-yard-long end zones were created, and teams were given four downs instead of three to advance the ball 10 yards.<sup class="reference" id="cite_ref-Danzig1956_13-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[13]  The roughing-the-passer penalty was implemented in 1914, and eligible players were first allowed to catch the ball anywhere on the field in 1918.<sup class="reference" id="cite_ref-14" style="line-height:1em;unicode-bidi:-webkit-isolate;">[14]

The professional era
William "Pudge" Heffelfinger, widely regarded as the first professional football player<p style="line-height:19.1875px;font-family:sans-serif;">The first instance of professional play in football was on November 12, 1892, when William "Pudge" Heffelfinger was paid $500 to play a game for the Allegheny Athletic Association in a match against the Pittsburgh Athletic Club. This is the first recorded instance of a player being paid to participate in a game of American football, although many athletic clubs in the 1880s offered to help players attain employment, gave out trophies or watches that players would pawn for money, or paid double in expense money. Football at the time had a strict sense of amateurism, and direct payment to players was frowned upon, if not outright illegal.<sup class="reference" id="cite_ref-birth_15-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[15]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">Professional play became common, and with it came rising salaries, unpredictable player movement, and the illegal use of amateur collegiate players in professional games. The National Football League, a group of professional teams that was originally established in 1920 as the American Professional Football Association, aimed to solve these problems. This new league's stated goals included an end to bidding wars over players, prevention of the use of college players, and abolition of the practice of paying players to leave another team.<sup class="reference" id="cite_ref-The_First_25_Years_16-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[16]  The NFL by 1922 had established itself as the premier professional football league.<sup class="reference" id="cite_ref-17" style="line-height:1em;unicode-bidi:-webkit-isolate;">[17]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">The dominant form of football at the time was played at the collegiate level, but the upstart NFL received a boost to its legitimacy in 1925 when an NFL team, the Pottsville Maroons, defeated a team of Notre Dame all-stars in an exhibition game.<sup class="reference" id="cite_ref-The_Curse_18-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[18]  A greater emphasis on the passing game helped professional football to further distinguish itself from the college game during the late 1930s.<sup class="reference" id="cite_ref-The_First_25_Years_16-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[16]  Football in general became increasingly popular following the 1958 NFL Championship game, a match between the Baltimore Colts and the New York Giants that is still referred to as the "Greatest Game Ever Played". The game, a 23–17 overtime victory by the Colts, was seen by millions of television viewers and had a major impact on the popularity of the sport. This helped football to become the most popular sport in the United States by the mid-1960s.<sup class="reference" id="cite_ref-Greatest_game_ever_played_19-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[19]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">A rival, the American Football League (AFL), arose in 1960 and challenged the NFL's dominance. The AFL began in relative obscurity but survived for several years due to a television contract with the ABC network. Competition for players heated up in 1965, when the AFL New York Jets signed rookie Joe Namath to a then-record US $437,000 contract. A five-year, $40 million NBC television contract followed, which helped to sustain the young league. The bidding war for players ended in 1966, when the two leagues agreed on a merger that would take full effect in 1970. This agreement provided for a common draft that would take place each year, and it instituted an annual championship game to be played between the champions of each league. That game began play in 1966 and came to be known as the Super Bowl.<sup class="reference" id="cite_ref-The_Second_25_Years_20-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[20]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">College football maintained a tradition of postseason bowl games. Each bowl game would be associated with a particular conference, and earning a spot in a bowl game was the reward for winning a conference. This arrangement was profitable, but it tended to prevent the two top-ranked teams from meeting in a true national championship game, as they would normally be committed to the bowl games of their respective conferences. Several systems have been used since 1992 to determine a national champion of college football. The first was the Bowl Coalition, in place from 1992 to 1994. This was replaced in 1995 by the Bowl Alliance, which gave way in 1997 to theBowl Championship Series (BCS).<sup class="reference" id="cite_ref-BCS_21-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[21]  The BCS arrangement has been controversial, and will be replaced in 2014 by a four-team playoff system.<sup class="reference" id="cite_ref-Presidents_get_playoff_plan_right_22-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[22]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">American football is today the most popular sport in the United States. In a 2013 poll conducted by Harris Interactive, professional and college football were the first and third most popular sports, respectively, and 45% of participants ranked some form of the game as their favorite sport.<sup class="reference" id="cite_ref-The_Harris_Poll:_NFL_continues_47-year_run_as_America.E2.80.99s_most_popular_sport_23-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[23]  The Super Bowl is the most popular single-day sporting event in the United States<sup class="reference" id="cite_ref-The_Second_25_Years_20-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[20]  and is among the biggest club sporting events in the world.<sup class="reference" id="cite_ref-Elite_clubs_on_Uefa_gravy_train_as_Super_Bowl_knocked_off_perch_24-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[24]

Etymology and names
<p style="line-height:19.1875px;font-family:sans-serif;">In the United States, American football is known simply as 'football'.<sup class="reference" id="cite_ref-Oxford_2-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[2]  The term 'football' was established in the rulebook for the 1876 college football season, when the sport first shifted from soccer-style rules to rugby-style rules. The sport could easily have been known as 'rugby' at this point, but Harvard, one of the primary proponents of the rugby-style game, compromised and did not request the sport's name be changed to 'rugby'.<sup class="reference" id="cite_ref-25" style="line-height:1em;unicode-bidi:-webkit-isolate;">[25]  In countries where other codes of football are popular, such as the United Kingdom or Australia, the terms 'gridiron' or 'American football' are favored.<sup class="reference" id="cite_ref-.27In_the_six.27_and_football.27s_other_strange_Americanisms_26-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[26] <sup class="reference" id="cite_ref-Living_off_the_grid_-_American_football_in_coastal_Australia_27-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[27]

Teams and positions
Main article: American football positions<p style="line-height:19.1875px;font-family:sans-serif;">A game is played between two teams of 11 players each.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._21_28-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[28] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._15_29-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[29] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._11_30-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[30]  It is legal to have fewer players on the field,<sup class="reference" id="cite_ref-LiveStrong_31-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[31]  but playing with more on the field is punishable by a penalty.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._21_28-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[28] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._107_32-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[32] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._71-72_33-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[33]  Teams may substitute any number of their players between downs;<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._21-22_34-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[34] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._53-54_35-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[35] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._45-46_36-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[36]  this "platoon" system replaced the original system, which featured limited substitution rules. The "platoon" system has resulted in teams utilizing specialized offensive, defensive and special teams squads.<sup class="reference" id="cite_ref-The_innovator_37-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[37]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">Individual players in a football game must be designated with a uniform number between 1 and 99. NFL teams are required to number their players by a league-approved numbering system, and any exceptions must be approved by the Commissioner.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._21_28-2" style="line-height:1em;unicode-bidi:-webkit-isolate;">[28]  NCAA and NFHS teams are "strongly advised" to number their offensive players according to a league-suggested numbering scheme.<sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._21-22_38-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[38] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._16-17_39-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[39]

Offensive unit
<p style="line-height:19.1875px;font-family:sans-serif;">The role of the offensive unit is to advance the ball down the field with the ultimate goal of scoring a touchdown.<sup class="reference" id="cite_ref-NFL_in_a_nutshell_40-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[40] A diagram of a typical pre-snap formation. The offense (red) is lined up in a variation of the I formation, while the defense (blue) is lined up in the 4-3 defense. Both formations are legal.<p style="line-height:19.1875px;font-family:sans-serif;">The offensive team must line up in a legal formation before they can snap the ball. An offensive formation is considered illegal if there are more than four players in the backfield or fewer than five players numbered 50-79 on the offensive line.<sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._15_29-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[29] <sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._21-24_41-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[41] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._57-58_42-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[42]  Players can temporarily line up in a position whose eligibility is different than their number permits as long as they immediately report the change to the referee, who then informs the defensive team of the change.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._36.2C_40_43-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[43]  Neither team's players, with the exception of the snapper, are allowed to line up in or cross the neutral zone until the ball is snapped. Interior offensive linemen are not allowed to move until the snap of the ball.<sup class="reference" id="cite_ref-Common_Penalties_in_American_Football_44-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[44]

Backs and backfield
A quarterback under center, ready to take the snap<p style="line-height:19.1875px;font-family:sans-serif;">The main backfield positions are the quarterback(QB), halfback/tailback (HB/TB) and fullback(FB). The quarterback is the leader of the offense. Either he or a coach calls the plays. Quarterbacks typically inform the rest of the offense of the play in the huddle before the team lines up. The quarterback lines up behind the center to take the snap and then hands the ball off, throws it or runs with it.<sup class="reference" id="cite_ref-NFL_in_a_nutshell_40-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[40] <sup class="reference" id="cite_ref-NFL_in_a_nutshell_40-2" style="line-height:1em;unicode-bidi:-webkit-isolate;">[40]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">The primary role of the halfback, who is also known as the tailback, is to carry the ball on running plays. Halfbacks may also serve as receivers. Fullbacks tend to be larger than halfbacks and function primarily as blockers, but they are sometimes used as runners in short-yardage situations<sup class="reference" id="cite_ref-Football_Players.27_Roles_in_Team_Offense_and_Defense_45-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[45]  and often are not used in passing situations.<sup class="reference" id="cite_ref-Fullbacks_back_en_vogue_46-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[46]

Linemen
<p style="line-height:19.1875px;font-family:sans-serif;">The offensive line consists of several players whose primary function is blocking. The leader of the offensive line is the center (C), who is responsible for snapping the ball to the quarterback<sup class="reference" id="cite_ref-Football_Players.27_Roles_in_Team_Offense_and_Defense_45-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[45]  and for making sure that the other linemen do their jobs during the play.<sup class="reference" id="cite_ref-Centers:_The_Unsung_Heroes_of_Football_47-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[47]  On either side of the center are the guards (G), while tackles (T) line up outside of the guards.<sup class="reference" id="cite_ref-Football_Players.27_Roles_in_Team_Offense_and_Defense_45-2" style="line-height:1em;unicode-bidi:-webkit-isolate;">[45]

Receivers
Washington Redskins wide receiver Santana Moss catches a pass over two Atlanta Falconsdefenders.<p style="line-height:19.1875px;font-family:sans-serif;">The principal receivers are the wide receivers (WR) and the tight ends (TE).<sup class="reference" id="cite_ref-Football.27s_Offensive_Team:_The_Receivers_48-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[48]  Wide receivers line up on or near the line of scrimmage, split outside of the line. The main goal of the wide receiver is to catch passes thrown by the quarterback,<sup class="reference" id="cite_ref-Football_Players.27_Roles_in_Team_Offense_and_Defense_45-3" style="line-height:1em;unicode-bidi:-webkit-isolate;">[45]  but they may also function as decoys or as blockers during running plays. Tight ends line up outside of the tackles and function both as receivers and as blockers.<sup class="reference" id="cite_ref-Football_Players.27_Roles_in_Team_Offense_and_Defense_45-4" style="line-height:1em;unicode-bidi:-webkit-isolate;">[45]

Defensive unit
Dallas Cowboys defensive players forceHouston Texans running back Arian Fosterto fumble the ball.<p style="line-height:19.1875px;font-family:sans-serif;">The role of the defense is to prevent the offense from scoring by tackling the ball carrier or by forcing turnovers(interceptions or fumbles).<sup class="reference" id="cite_ref-NFL_in_a_nutshell_40-3" style="line-height:1em;unicode-bidi:-webkit-isolate;">[40]

Defensive line
<p style="line-height:19.1875px;font-family:sans-serif;">The defensive line consists of defensive ends (DE) anddefensive tackles (DT). Defensive ends line up on the end of the line, while defensive tackles line up on the inside. Their primary responsibilities are to stop running plays on the inside and outside, respectively, as well as to pressure the quarterback on passing plays and to occupy the line so that the linebackers can break through.<sup class="reference" id="cite_ref-Football_Players.27_Roles_in_Team_Offense_and_Defense_45-5" style="line-height:1em;unicode-bidi:-webkit-isolate;">[45]

Linebackers
<p style="line-height:19.1875px;font-family:sans-serif;">Linebackers line up behind the defensive line but in front of the defensive backfield. They are divided into two types: middle linebackers (MLB) and outside linebackers (OLB). Linebackers are the defensive leaders and call the defensive plays. Their diverse roles include defending the run, pressuring the quarterback, and guarding backs, wide receivers and tight ends in the passing game.<sup class="reference" id="cite_ref-Football.27s_Defensive_Team:_The_Linebackers_49-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[49]

Defensive backfield
Cornerback Brent Grimes of theHamburg Sea Devils intercepts a pass.<p style="line-height:19.1875px;font-family:sans-serif;">The defensive backfield, often called the "secondary," consists of cornerbacks (CB) andsafeties (S). Safeties are themselves divided into free safeties (FS) and strong safeties (SS).<sup class="reference" id="cite_ref-Football_Players.27_Roles_in_Team_Offense_and_Defense_45-6" style="line-height:1em;unicode-bidi:-webkit-isolate;">[45]  Cornerbacks line up outside the defensive formation and cover wide receivers, while safeties line up between the cornerbacks but farther back in the secondary. The defensive backfield is responsible for stopping deep passes and running plays.<sup class="reference" id="cite_ref-Football_Players.27_Roles_in_Team_Offense_and_Defense_45-7" style="line-height:1em;unicode-bidi:-webkit-isolate;">[45]

Special teams unit
<p style="line-height:19.1875px;font-family:sans-serif;">The special teams unit executes and defends punts and kicking plays.<sup class="reference" id="cite_ref-NFL_in_a_nutshell_40-4" style="line-height:1em;unicode-bidi:-webkit-isolate;">[40]

Field goals and kickoffs
Kicker Jeff Reed of the Pittsburgh Steelers executes a kickoff.<p style="line-height:19.1875px;font-family:sans-serif;">Three positions are specific to the field goal and PAT (point after touchdown) unit: the placekicker (K or PK), holder (H) and long snapper(LS). The long snapper's job is to snap the ball to the holder, who will catch the ball and position it for the placekicker. On kickoffs, the ball is kicked off of a tee. The player on the receiving team who catches the ball is the "kickoff returner" (KR).<sup class="reference" id="cite_ref-The_Role_of_Special_Teams_in_a_Football_Game_50-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[50]

Punts and punt returners
<p style="line-height:19.1875px;font-family:sans-serif;">The positions specific to punt plays are the punter (P), long snapper and gunners. The long snapper snaps the ball directly to the punter, who then drops the ball and kicks it. Gunners line up outside of the line and race down the field to tackle the player on the receiving team who catches the ball, known as the "punt returner" (PR).<sup class="reference" id="cite_ref-Football_Special_Teams:_Players_on_a_Punt_Team_51-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[51]

Equipment
See also: American football protective equipmentA player wearing a helmet. Shoulder pads and thigh pads are visible under his uniform<p style="line-height:19.1875px;font-family:sans-serif;">American football is a violent game, and players wear armor-like padding to protect themselves. At minimum, players wear a protective football helmet, which has a face mask, and a set of shoulder pads. Additional padding may be required, depending on the league. This may include thigh pads and guards, knee pads, chest protectors, and mouth guards.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._24-27_52-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[52] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._22_53-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[53] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._17-19_54-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[54]  College and high school play require the use of a mouthguard.<sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._22_53-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[53] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._17-19_54-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[54]  Despite these protections, injuries do occur - particularly concerning are head injuries such as concussions.<sup class="reference" id="cite_ref-Head_Injuries_in_Football_55-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[55]  Concussions are often caused by helmet-to-helmet or upper-body contact between opposing players, although the helmets have prevented more serious injuries such as skull fractures.<sup class="reference" id="cite_ref-Can_Football_Finally_Tackle_Its_Injury_Problem.3F_56-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[56]

Rules
Main article: American football rulesA player (dark jersey) scores a touchdown while a defender (in white) looks on. The goal line is marked by the small orange pylon.<p style="line-height:19.1875px;font-family:sans-serif;">The rules of American football vary somewhat from league to league, but each level of the sport has a prominent, national body that determines a unified code of rules for that level of play. The National Football League (NFL) is the highest level of professional football in the United States.<sup class="reference" id="cite_ref-NFL_rules_57-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[57]  TheNational Collegiate Athletic Association (NCAA), which manages university athletics in the United States for most colleges and universities, maintains the rules for college football,<sup class="reference" id="cite_ref-NCAA_Rulebook_58-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[58]  and high school football is overseen by the National Federation of State High School Associations (NFHS).<sup class="reference" id="cite_ref-59" style="line-height:1em;unicode-bidi:-webkit-isolate;">[59]  The largest organization for youth football leagues, whose players are younger than high school age, is the Pop Warner Little Scholars.<sup class="reference" id="cite_ref-60" style="line-height:1em;unicode-bidi:-webkit-isolate;">[60]

Scoring
<p style="line-height:19.1875px;font-family:sans-serif;">There are several ways to score in American football. The touchdown, worth six points, is the most valuable scoring play. A touchdown is scored when the ball is advanced into or caught in, or when a ball in play is recovered in, the end zone of the opposing team.<sup class="reference" id="cite_ref-NFL_in_a_nutshell_40-5" style="line-height:1em;unicode-bidi:-webkit-isolate;">[40]  The scoring team then attempts a try, more commonly known as the point(s)-after-touchdown (PAT), which is a single scoring opportunity. A PAT is attempted from the two- or three-yard line, depending on the level of play. If scored by a placekick, it is worth one point and is called the extra point.<sup class="reference" id="cite_ref-Duke_61-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[61]  If it is scored by what would normally be a touchdown, it is called the two-point conversion<sup class="reference" id="cite_ref-Duke_61-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[61]  and is worth two extra points. No extra points are awarded on a failed attempt.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._56-57_62-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[62] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._77-79_63-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[63] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._65-66_64-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[64]  The extra point is almost always successful at the professional level and slightly less so at the amateur level, while the two-point conversion has a significantly lower success rate.<sup class="reference" id="cite_ref-Duke_61-2" style="line-height:1em;unicode-bidi:-webkit-isolate;">[61]  A field goal, worth three points, is scored when the ball is placekicked or, very rarely, dropkicked through the uprights and over the crossbars of the defense's goalposts.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._57-59_65-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[65] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._79-80_66-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[66] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._66_67-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[67]  After a PAT attempt or successful field goal the scoring team must kick the ball off to the other team.<sup class="reference" id="cite_ref-Beginner.27s_Guide_to_Football_68-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[68]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">A safety is worth two points and is scored by the defense. It occurs when the ball carrier is tackled in his own end zone.<sup class="reference" id="cite_ref-NFL_in_a_nutshell_40-6" style="line-height:1em;unicode-bidi:-webkit-isolate;">[40]  The team that scores the safety then receives the ensuing kickoff.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._60_69-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[69]

Field
A football field as seen from behind one end zone. The tall, yellow goal posts mark where the ball must pass for a successful field goal or extra point. The large, rectangular area marked with the team name is the end zone.<p style="line-height:19.1875px;font-family:sans-serif;">Football games are played on a rectangular field that measures 120 yards (109.7 meters) long and 53.3 yards (160 ft (49 m)) wide. Lines marked along the ends and sides of the field are known respectively as the end lines and side lines, and goal lines are marked 10 yd (9.1 m) outward from each end line. Weighted pylons are placed on the inside corner of the intersections of the goal lines and end lines.

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">White markings on the field identify the distance from the end zone. Inbound lines, or hash marks, are short parallel lines that mark off one-yard increments. Yard lines, which run the width of the field, are marked every five yards. A line one yard wide is placed at each end of the field. This line is marked at the center of the two-yard line in professional play and at the three-yard line in college play. Numerals that display the yard lines in multiples of ten are placed along both sides of the field.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._v.2C_1_70-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[70] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._18-19.2C_23-24_71-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[71] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._11-12.2C_13.2C_28_72-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[72]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">Goalposts are at the center of the plane of each of the two end lines. The crossbar of these posts is ten feet above the ground, with vertical uprights at the end of the crossbar 18 ft 6 in (5.64 m) apart for professional and collegiate play and 23 ft 4 in (7.11 m) for high school.<sup class="reference" id="cite_ref-Sonar_Domes_73-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[73] The uprights extend vertically 30 ft (9.1 m) on professional fields, a minimum of 30 feet on college fields, and a minimum of 10 ft (3.0 m) on high school fields. Goal posts are padded at the base, and orange ribbons are normally placed at the tip of each upright.<sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._18_74-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[74] <sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._2_75-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[75] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._14_76-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[76]

Duration and time stoppages
<p style="line-height:19.1875px;font-family:sans-serif;">Football games last for a total of 60 minutes in professional and college play and are divided into two halves of 30 minutes and four quarters of 15 minutes.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._14_77-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[77] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._45_78-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[78]  High school football games are 48 minutes in length with two halves of 24 minutes and four quarters of 12 minutes.<sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._38-39_79-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[79]  The two halves are separated by a halftime period, and the first and third quarters are also followed by a short break.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._14_77-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[77] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._45_78-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[78] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._39_80-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[80]  Teams switch goals following the first and third quarters.<sup class="reference" id="cite_ref-How_Football_Game_Time_Is_Measured_in_Quarters_81-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[81]  If a down is in progress when a quarter ends, play continues until the down is completed.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._14-18_82-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[82] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._47-53_83-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[83] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._38-45_84-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[84]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">Games last longer than their defined length due to play stoppages, so that the average NFL game lasts just over three hours.<sup class="reference" id="cite_ref-USA_Today_85-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[85]  Time in a football game is measured by the game clock. An operator is responsible for starting, stopping and operating the game clock based on the direction of the appropriate official.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._14_77-2" style="line-height:1em;unicode-bidi:-webkit-isolate;">[77] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._16.2C_41_86-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[86]  A separate clock, the play clock, is used to determine if a delay of gameinfraction has been committed. If the play clock expires before the ball has been snapped or free-kicked, a delay of game foul is called on the offense. The play clock is set to 40 seconds in professional and college football and to 25 seconds in high school play or following certain administrative stoppages in the former levels of play.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._14-18_82-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[82] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._41.2C_46-47_87-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[87] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._36.2C_45_88-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[88]

Advancing the ball and downs
Carolina Panthers quarterback Jake Delhomme (number 17) in the motion of throwing a forward pass.<p style="line-height:19.1875px;font-family:sans-serif;">There are two main ways that the offense can advance the ball: running and passing. In a typical play, the quarterback calls the play, and the center passes the ball backwards and under his legs to the quarterback in a process known as the snap. The quarterback then either hands the ball off to a back, throws the ball or runs with it himself. The play ends when the player with the ball is tackled or goes out of bounds or a pass hits the ground without a player having caught it.<sup class="reference" id="cite_ref-Beginner.27s_Guide_to_Football_68-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[68]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">The offense is given a series of four plays, known as downs. If the offense advances ten or more yards in the four downs, they are awarded a new set of four downs. If they fail to advance ten yards, possession of the ball is turned over to the defense. Normally, if the offense reaches their fourth down they will punt the ball to the other team, which forces them to begin their drive from further down the field.<sup class="reference" id="cite_ref-Beginner.27s_Guide_to_Football_68-2" style="line-height:1em;unicode-bidi:-webkit-isolate;">[68]  A group of officials, the chain crew, keeps track of both the downs and the distance measurements.<sup class="reference" id="cite_ref-The_Orchestration_of_the_Chain_Gang_89-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[89]  On television, a yellow line is electronically superimposed on the field to show the first down line to the viewing audience.<sup class="reference" id="cite_ref-Yellow_line_90-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[90]

Kicking
Green Bay Packers Placekicker Mason Crosby attempts a field goal by kicking the ball from the hands of a holder. This is the standard method to score field goals or extra points.<sup class="reference" id="cite_ref-PFRA_91-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[91] <sup class="reference" id="cite_ref-The_last_dropkick_92-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[92] <p style="line-height:19.1875px;font-family:sans-serif;">There are two categories of kicks in football: scrimmage kicks, which can be executed by the offensive team on any down from behind or on the line of scrimmage,<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._50_93-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[93] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._34_94-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[94] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._32_95-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[95]  and free kicks.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._6_96-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[96] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._30_97-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[97] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._27_98-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[98]  The free kicks are the kickoff, which starts the first and third quarters and overtime and follows a try attempt or a successful field goal, and the safety kick, which follows a safety.<sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._34_94-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[94] <sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._8-9_99-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[99] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._31-32_100-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[100]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">On a kickoff, the ball is placed at the 35-yard line of the kicking team in professional and college play and at the 40-yard line in high school play. The ball may be drop-kicked or place-kicked. If a place kick is chosen, the ball can be placed on the ground or on a tee, and a holder may be used in either case. On a safety kick, the kicking team kicks the ball from their own 20-yard line. They can punt, drop-kick or place-kick the ball, but a tee may not be used in professional play. Any member of the receiving team may catch or advance the ball, and the ball may be recovered by the kicking team once it has gone at least ten yards or has been touched by any member of the receiving team.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._29-30_101-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[101] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._61-64_102-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[102] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._15.2C_46.2C_52-53_103-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[103]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">Place kicks and drop kicks can be used to score points from scrimmage,<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._57-59_65-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[65] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._79-80_66-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[66] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._66_67-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[67]  but the place kick is the standard method used<sup class="reference" id="cite_ref-PFRA_91-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[91]  due to the difficulty of reliably dropkicking a football.<sup class="reference" id="cite_ref-PFRA_91-2" style="line-height:1em;unicode-bidi:-webkit-isolate;">[91] <sup class="reference" id="cite_ref-The_last_dropkick_92-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[92]  Once the ball has been kicked from a scrimmage kick, it can be advanced by the kicking team only if it is caught or recovered behind the line of scrimmage. If it is touched or recovered by the kicking team beyond this line, it becomes dead at the spot where it was touched.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._33-34.2C_50-53_104-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[104] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._55-56.2C_63-64_105-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[105] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._49.2C_53-54_106-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[106]  The kicking team is prohibited from interfering with the receiver's opportunity to catch the ball, and the receiving team has the option of signaling for a fair catch. This prohibits the defense from blocking into or tackling the receiver, but the play ends as soon as the ball is caught and the ball may not be advanced.<sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._7_107-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[107] <sup class="reference" id="cite_ref-NCAA_Rules_2011-2012.2C_p._30.2C_66-67_108-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[108] <sup class="reference" id="cite_ref-NFHS_Rules_2012.2C_p._2756.2C_109-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[109]

Officials and fouls
See also: Official (American football), Chain crew, and Penalty (gridiron football)Officials discuss a call on the field.Officials use the chains to measure for a first down. Here, the ball is just short of the pole and therefore short of a first down.<p style="line-height:19.1875px;font-family:sans-serif;">Officials are responsible for enforcing game rules and monitoring the clock. All officials carry awhistle and wear black-and-white striped shirts and black hats except for the referee, whose hat is white. Each carries a weighted yellow flag that is thrown to the ground to signal that a penaltyhas been called. An official who spots multiple penalties will throw his hat as a secondary signal.<sup class="reference" id="cite_ref-American_Football_Officials_110-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[110]  The seven officials on the field are each tasked with a different set of responsibilities:<sup class="reference" id="cite_ref-American_Football_Officials_110-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[110] A modern down indicator box is mounted on a pole and is used to mark the current line of scrimmage. The number on the marker is changed using a dial.*The line judge supervises player substitutions, the line of scrimmage during punts, and game timing. He notifies the referee when time has expired at the end of a quarter and notifies the head coach of the home team when five minutes remain for halftime. In the NFL, the line judge also alerts the referee when two minutes remain in the half. If the clock malfunctions or becomes inoperable, the line judge becomes the official timekeeper. <p style="line-height:19.1875px;font-family:sans-serif;">Another set of officials, the chain crew, hold the chains that measure the 10 yards needed for a first down. The chain crew moves the chains up and down the field and keeps track of the current down using the down marker. This system has been used for over 100 years, is considered to be an accurate measure of distance, and is rarely subject to criticism from either side.<sup class="reference" id="cite_ref-The_Orchestration_of_the_Chain_Gang_89-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[89]
 * The referee is charged with oversight and control of the game and is the authority on the score, the down number, and any and all rule interpretations in discussions between the other officials. He announces all penalties and discusses the infraction with the offending team's captain, monitors for illegal hits against the quarterback, makes requests for first-down measurements, and notifies the head coach whenever a player is ejected.
 * The umpire, who is positioned in the backfield, watches play along the line of scrimmage to make sure that no more than 11 offensive players are on the field prior to the snap and that no offensive linemen are illegally downfield on pass plays. He monitors the contact between offensive and defensive linemen and calls most of the holding penalties. The umpire records the number of timeouts taken and the winner of the coin toss and the game score, assists the referee in situations involving possession of the ball close to the line of scrimmage, determines whether player equipment is legal, and dries wet balls prior to the snap if a game is played in rain.
 * The head linesman lines up opposite the line judge. He watches for any line-of-scrimmage and illegal use-of-hands violations and assists the line judge with illegal shift or illegal motion calls. The head linesman also rules on out-of-bounds calls that happen on his side of the field, oversees the chain crew and marks the forward progress of a runner when a play has been whistled dead.
 * The back judge ensures that the defensive team has no more than 11 players on the field and determines whether catches are legal, whether field goal or extra point attempts are good, and whether a pass interference violation occurred.
 * The side judge mainly duplicates the functions of the back judge but is positioned twenty yards downfield from the line of scrimmage and opposite to the back judge.
 * The field judge monitors and controls the play clock, counts the number of defensive players on the field, and watches for offensive pass interference and illegal use-of-hands violations by offensive players. He also makes decisions regarding catches, recoveries, the ball spot when a player goes out of bounds, and illegal touching of fumbled balls that have crossed the line of scrimmage.

Gameplay
Main article: American football rulesTwo teams lined up on either side of theline of scrimmage. The center of the offensive team (right) holds the ball before the snap.<p style="line-height:19.1875px;font-family:sans-serif;">American football is played between two teams of 11 players each and uses an oval ball in the shape of a prolate spheroid.<sup class="reference" id="cite_ref-Oxford_2-2" style="line-height:1em;unicode-bidi:-webkit-isolate;">[2] <sup class="reference" id="cite_ref-NFL_Rules_2012.2C_p._3_111-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[111]  The game is played between two teams, one of which possesses the ball. The team in control of the ball, termed the offense, tries to advance the ball toward the end zone, the scoring area at either end of the field. The other team, termed the defense, tries to prevent the offense from advancing the ball.<sup class="reference" id="cite_ref-Beginner.27s_Guide_to_Football_68-3" style="line-height:1em;unicode-bidi:-webkit-isolate;">[68]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">A coin toss prior to the start of the game determines which team will have first possession of the ball. The game begins with a kickoff, in which the placekicker of the kicking team kicks the ball from a tee. A player from the receiving team catches the ball and runs towards the opposing end zone until he is tackled or steps out of bounds, at which point the ball is considered dead. The spot at which the ball becomes dead becomes the first line of scrimmage, and play begins from that point.

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">An official then places the ball on the field so that the long axis of the ball is parallel to the sidelines. It is placed at the middle of the field, between the two sets of hash marks, at the same distance from the goal line as where the previous play was declared dead. The teams line up on either side of the line of scrimmage or "neutral zone," which is a space that extends from sideline to sideline and is as wide as the length of the ball. Neither team may cross the neutral zone before the start of the play.

Leagues and tournaments
<p style="line-height:19.1875px;font-family:sans-serif;">The National Football League (NFL) and the National Collegiate Athletic Association (NCAA) are the most popular football leagues in the United States.<sup class="reference" id="cite_ref-Oregon_Live_112-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[112]  The National Football League (NFL) was founded in 1920<sup class="reference" id="cite_ref-NFL_founded_in_Canton_113-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[113]  and has since become the largest and most popular sport in the United States.<sup class="reference" id="cite_ref-China_fast_catching_American_football_fever_114-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[114]  The NFL has the highest average attendance of any sporting league in the world, with an average attendance of almost 70,000 persons during the 2011 NFL Season.<sup class="reference" id="cite_ref-And_the_silver_goes_to..._115-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[115]  The NFL championship game, the Super Bowl, is among the biggest events in club sports worldwide.<sup class="reference" id="cite_ref-Elite_clubs_on_Uefa_gravy_train_as_Super_Bowl_knocked_off_perch_24-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[24]  It is played between the champions of the National Football Conference (NFC) and theAmerican Football Conference (AFC), and its winner is awarded the Vince Lombardi Trophy.<sup class="reference" id="cite_ref-Let.27s_Learn_About:_The_Vince_Lombardi_Trophy_116-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[116]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">Collegiate football (NCAA) ranks third in overall popularity in the United States, after baseball.<sup class="reference" id="cite_ref-Harris_Poll_2012_117-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[117]  The NCAA is divided into three Divisions: Division I, Division II and Division III.<sup class="reference" id="cite_ref-About_the_NCAA_118-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[118]  Division I football is further divided into two subdivisions: the Football Bowl Subdivision (FBS) and the Football Championship Subdivision (FCS).<sup class="reference" id="cite_ref-Differences_Among_the_Three_Divisions:_Division_I_119-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[119]  The champions of Division I-FCS, Division II and Division III are determined through playoff systems, and the Division I-FBS champion is determined through the Bowl Championship Series (BCS). Division I-FBS will switch to a four-team playoff system in 2014.<sup class="reference" id="cite_ref-Postseason_Football_120-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[120]

Minor professional leagues
<p style="line-height:19.1875px;font-family:sans-serif;">Several football leagues have been formed as rival leagues to the NFL. The XFL was created in 2001 by Vince McMahon and lasted for only one season. Despite television contracts with NBC and UPN and high expectations, the XFL suffered from poor television ratings and a low quality of play.<sup class="reference" id="cite_ref-No_More_Springtimes_for_the_XFL_as_League_Folds_121-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[121]  The United Football League (UFL) has suspended its 2012 season, its fourth, due to financial issues.<sup class="reference" id="cite_ref-Just_What_Is_Going_on_with_the_UFL.3F_122-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[122] The United States Football League (USFL) operated for three seasons from 1983-1985 but collapsed due to poor business decisions and monetary problems. A subsequent USD $1.5 billion antitrust lawsuit against the NFL was successful in court, but the league was awarded only three dollars in damages.<sup class="reference" id="cite_ref-Twenty_years_later.2C_USFL_still_brings_fond_memories_123-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[123]  The World Football League (WFL) played for two seasons, in 1974 and 1975, but faced monetary issues so severe that the league could not pay its players. In its second and final season the WFL attempted to establish a stable credit rating, but the league disbanded before its second season could be completed.<sup class="reference" id="cite_ref-The_Day_The_Money_Ran_Out_124-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[124]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">The most successful minor league was the American Football League, which existed from 1960 to 1969. The AFL operated as a minor league from 1960 to 1964 and then signed a five-year, USD $36 million television deal with NBC and competed directly against the NFL. AFL teams signed NFL players to contracts, and their popularity grew to challenge that of the NFL. The two leagues merged in the1970 season, and all AFL teams joined the NFL. An earlier minor league, the All-America Football Conference, was in play from 1946 to 1949. Two AAFC teams, the Cleveland Browns and the San Francisco 49ers, became members of the NFL.<sup class="reference" id="cite_ref-Off-the-field_competition_yields_game-changing_merger_125-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[125]

International play
Players with one of the youth divisions of theBorregos Salvajes football program of theMonterrey Institute of Technology and Higher Education, Mexico City.<p style="line-height:19.1875px;font-family:sans-serif;">Football leagues exist throughout the world, but the game has yet to achieve the international success and popularity of baseball and basketball.<sup class="reference" id="cite_ref-126" style="line-height:1em;unicode-bidi:-webkit-isolate;">[126]  NFL Europa, thedevelopmental league of the NFL, operated from 1991 to 1992 and then from 1995 to 2007. At the time of its closure, NFL Europe had five teams based in Germany and one in the Netherlands.<sup class="reference" id="cite_ref-NFL_Europa_to_cease_operations_127-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[127]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">The European Football League (EFL), run by the European Federation of American Football (EFAF), is an annual invitational tournament between the champions or co-champions of competitions run by EFAF members. The league's championship game is the Eurobowl.<sup class="reference" id="cite_ref-European_Football_League_EFL_128-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[128]  Other EFAF tournaments include the EFAF Cup, played between the top teams from national leagues in a similar manner to the UEFA Cup,<sup class="reference" id="cite_ref-EFAF_Cup_129-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[129]  theAtlantic Cup, played between teams from the Atlantic region of Europe,<sup class="reference" id="cite_ref-Atlatic_Cup_130-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[130]  and theChallenge Cup, played between teams from newer federations that are not eligible to play in the EFL or EFAF Cup.<sup class="reference" id="cite_ref-Challenge_Cup_131-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[131]  The International Federation of American Football(IFAF), an international body of American football federations, runs tournaments such as the IFAF World Championship, which is held every four years since 1999, the IFAF Women's World Championship, the IFAF U-19 World Championship and the Flag Football World Championship. The IFAF also organizes the annual International Bowl game.<sup class="reference" id="cite_ref-Championship_Competitions_132-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[132] Football federations are also present in Asia, Oceania, and Pan America, and a total of 64 national football federations exist as of July 2012. At the international level, Canada, Mexico, and Japan are considered to be second-tier, while Austria, Germany, and Francewould rank among a third tier. All of these countries rank far below the United States, which is dominant at the international level.<sup class="reference" id="cite_ref-Football_in_Olympics_is_a_dream_that_could_become_a_reality_133-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[133]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">Football is not an Olympic sport, but it was a demonstration sport at the 1932 Summer Olympics.<sup class="reference" id="cite_ref-Football_remains_an_Olympic_long_shot_1-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[1]  The IFAF seeks recognition from the International Olympic Committee (IOC), which would be the second-to-last step to gaining admission to the Olympic Games themselves. It would take at least a decade to be admitted.<sup class="reference" id="cite_ref-Football_in_Olympics_is_a_dream_that_could_become_a_reality_133-1" style="line-height:1em;unicode-bidi:-webkit-isolate;">[133]  Several major obstacles hinder the IFAF goal of achieving status as an Olympic sport: the predominant participation of men in international play, and the short three-week Olympic schedule. Large team sizes are an additional difficulty, due to the Olympics' set limit of 10,500 athletes and coaches. These issues are similar to those that rugby union faced prior to being admitted into the Olympics in the form of rugby sevens, a modified version of the sport, but football also has the issue of global visibility. Nigel Melville, the CEO of USA Rugby, noted that while rugby union has a major international presence through the International Rugby Board, "American football is recognized globally as a sport, but it's not played globally". In order to solve these concerns, major effort has been put into promoting flag football, a modified version of American football, at the international level.<sup class="reference" id="cite_ref-Football_in_Olympics_is_a_dream_that_could_become_a_reality_133-2" style="line-height:1em;unicode-bidi:-webkit-isolate;">[133]

Variations and related sports
See also: Canadian football, Arena football, and Flag footballMen playing a game of flag football<p style="line-height:19.1875px;font-family:sans-serif;">Canadian football is related to American football, as both share roots in rugby football. Although Canadian football developed independently from American football, the two games have similar rules. In the Canadian variant, the field measures 150 yards by 65 yards, and it has 20-yard end zones. The main differences in rules are that the Canadian game has three downs instead of four, which results in a game that is more pass-oriented; there are twelve players on each side instead of eleven, and a single point is scored if the offensive team kicks the ball out of the defense's end zone.<sup class="reference" id="cite_ref-Encyclop.C3.A6dia_Britannica_134-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[134]  The Canadian Football League (CFL) is the major Canadian league and is the second-most popular sporting league in Canada, behind only the National Hockey League.<sup class="reference" id="cite_ref-Head_to_Head:_CFL_vs._NFL_135-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[135]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">A major variant of American football is arena football, played by the Arena Football League(AFL).<sup class="reference" id="cite_ref-Arena_football:_Is_it_America.27s_fifth_major_sport.3F_136-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[136]  Arena football has eight-player teams and uses an indoor field 50 yards in length, excluding end zones, and 28.3 yards wide. Punting is illegal, and kickoffs are attempted from the goal line. Large overhead nets deflect forward passes and kicks that hit them, and deflected kicks are live balls that may be recovered by either team.<sup class="reference" id="cite_ref-Arena_football:_The_basics_137-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[137]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">Below the Arena Football League are what New York Times writer Mike Tanier described as the "most minor of minor leagues:" indoor football leagues. Indoor leagues such as the Indoor Football League, Southern Indoor Football League, Ultimate Indoor Football League,Continental Indoor Football League and American Professional Football League are unstable. Their franchises regularly move from one league to another, merge with other teams, or dissolve entirely.<sup class="reference" id="cite_ref-Staying_in_the_Game_on_Football.E2.80.99s_Fringe_138-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[138]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;font-family:sans-serif;">Flag football is a variation of American football in which players are not tackled, but a play is ruled dead once a "flag" attached the waist of the runner is pulled off.<sup class="reference" id="cite_ref-Flag_football_139-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[139]  Touch football is a similar variant in which a play ends when the runner is touched.<sup class="reference" id="cite_ref-Touch_football_140-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[140]  A game of touch football may require that the player be touched with either one hand or two, depending on the rules used.<sup class="reference" id="cite_ref-The_32_Rules_of_Thanksgiving_Touch_Football_141-0" style="line-height:1em;unicode-bidi:-webkit-isolate;">[141]