Etymology and namesIn the United States, American football is referred to as "football". The term "football" was officially established in the rulebook for the 1876 college football season, when the sport first shifted from soccer-style rules to rugby-style rules. Although it could easily have been called "rugby" at this point, Harvard, one of the primary proponents of the rugby-style game, compromised and did not request the name of the sport be changed to "rugby". The terms " gridiron" or "American football" are favored in English-speaking countries where other types of are popular, such as the United Kingdom, Ireland, New Zealand, and Australia.
Early historyAmerican football evolved from the sports of and . Rugby, like American football, is a sport where two competing teams vie for control of a ball, which can be kicked through a set of goalposts or run into the opponent's goal area to score points. What is considered to be the first American football game was played on November 6, 1869, between and , two college teams. They consisted of 25 players per team and used a round ball that could not be picked up or carried. It could, however, be kicked or batted with the feet, hands, head or sides, with the objective being to advance it into the opponent's goal. Rutgers won the game 6–4. Collegiate play continued for several years with matches played using the rules of the host school. Representatives of Yale, , Princeton and Rutgers met on October 19, 1873, to create a standard set of rules for use by all schools. Teams were set at 20 players each, and fields of were specified. Harvard abstained from the conference, as they favored a rugby-style game that allowed running with the ball. After playing using both Canadian and American rules, the Harvard players preferred the Canadian style of having only 11 men on the field, running the ball without having to be chased by an opponent, the forward pass, tackling, and using an oblong instead of a round ball. An 1875 Harvard–Yale game played under rugby-style rules was observed by two Princeton athletes who were impressed by it. They introduced the sport to Princeton, a feat the compared to "selling s to s". Princeton, Harvard, Yale, and Columbia then agreed to intercollegiate play using a form of rules with a modified scoring system. These schools formed the , although Yale did not join until 1879. Yale player , now regarded as the "Father of American Football", secured rule changes in 1880 that reduced the size of each team from 15 to 11 players and instituted the snap to replace the chaotic and inconsistent .
Evolution of the gameThe introduction of the snap resulted in an unexpected consequence. Prior to the snap, the strategy had been to punt if a scrum resulted in bad field position. However, a group of Princeton players realized that as the snap was uncontested, they could now hold the ball indefinitely to prevent their opponent from scoring. In 1881, in a game between Yale and Princeton, both teams used this strategy to maintain their undefeated records. Each team held the ball, gaining no ground, for an entire half, resulting in a 0–0 tie. This "block game" proved extremely unpopular with both teams' spectators and fans. A rule change was necessary to prevent this strategy from taking hold, and a reversion to the scrum was considered. However, Camp successfully proposed a rule in 1882 that limited each team to three downs, or tackles, to advance the ball . Failure to advance the ball the required distance within those three downs would result in control of the ball being forfeited to the other team. This change effectively made American football a separate sport from rugby, and the resulting five-yard lines added to the field to measure distances made it resemble a gridiron in appearance. Other major rule changes included a reduction of the field size to 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 a goal from the field. Additionally, tackling below the waist was legalized, and a static was instituted. Despite these new rules, football remained a violent sport. Dangerous mass-formations like the resulted in serious injuries and deaths. A 1905 peak of 19 fatalities nationwide resulted in a threat by President to abolish the game unless major changes were made. In response, 62 colleges and universities 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 renamed the (NCAA). The legal ._Other_rule_changes_introduced_that_year_included_the_reduction_of_playing_time_from_70_to_60_minutes_and_an_increase_of_the_distance_required_for_a_first_down_from_._To_reduce_infighting_and_dirty_play_between_teams,_the_Neutral_zone_(gridiron_football).html" ;"title="Harvard_Stadium#Impact_on_American_Football.html" "title="roup (disambiguation), group of individuals (human or non-human) working together to achieve their goal. As defined by Professor Leigh Thompson (academic), ...
Professional eraOn November 12, 1892, was paid $500 to play a game for the 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 indirect benefits, such as helping players attain employment, giving out trophies or watches that players could pawn for money, or paying double in expense money. Despite these extra benefits, the game had a strict sense of amateurism at the time, and direct payment to players was frowned upon, if not prohibited outright. Over time, professional play became increasingly common, and with it came rising salaries and unpredictable player movement, as well as the illegal payment of college players who were still in school. The (NFL), 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. By 1922, the NFL had established itself as America's premier professional football league. The dominant form of football at the time was played at the collegiate level. The upstart NFL received a boost to its legitimacy in 1925, however, when an NFL team, the , defeated a team of all-stars in an . A greater emphasis on the passing game helped professional football to distinguish itself further from the college game during the late 1930s. Football, in general, became increasingly popular following the 1958 NFL Championship game, a match between the and the 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, along with the innovations introduced by the new (AFL) in the early 1960s, helped football to become the most popular sport in the United States by the mid-1960s. The rival AFL arose in 1960 and challenged the NFL's dominance. The AFL began in relative obscurity but eventually thrived, with an initial television contract with the television network. The AFL's existence forced the conservative NFL to expand to Dallas and Minnesota in an attempt to destroy the new league. Meanwhile, the AFL introduced many new features to professional football in the United States: official time was kept on a scoreboard clock rather than on a watch in the referee's pocket, as the NFL did; optional two-point conversions by pass or run after touchdowns; names on the jerseys of players; and several others, including expansion of the role of minority players, actively recruited by the league in contrast to the NFL. The AFL also signed several star college players who had also been drafted by NFL teams. Competition for players heated up in 1965, when the AFL signed rookie to a then-record $437,000 contract (equivalent to $ in ). A five-year, $40 million television contract followed, which helped to sustain the young league. The bidding war for players ended in 1966 when NFL owners approached the AFL regarding a merger, and the two leagues agreed on one that took full effect in 1970. This agreement provided for a common draft that would take place each year, and it instituted an annual World Championship game to be played between the champions of each league. This championship game began play at the end of the 1966 season. Once the merger was completed, it was no longer a championship game between two leagues and reverted to the NFL championship game, which came to be known as the . College football maintained a tradition of postseason . Each bowl game was 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 , in place from 1992 to 1994. This was replaced in 1995 by the , which gave way in 1997 to the (BCS). The BCS arrangement proved to be controversial, and was replaced in 2014 by the (CFP).
Teams and positionsA football game is played between two teams of 11 players each.NFL Rules 2012, p. 21.NFHS Rules 2012, p. 11. Playing with more on the field is punishable by a penalty.NCAA Rules 2011–2012, p. 107.NFHS Rules 2012, pp. 71–72. Teams may substitute any number of their players between downs;NFL Rules 2012, pp. 21–22.NCAA Rules 2011–2012, pp. 53–54.NFHS Rules 2012, pp. 45–46. this "platoon" system replaced the original system, which featured limited substitution rules, and has resulted in teams utilizing specialized offensive, defensive and units. Individual players in a football game must be designated with a 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 . NCAA and NFHS teams are "strongly advised" to number their offensive players according to a league-suggested numbering scheme.NCAA Rules 2011–2012, pp. 21–22.NFHS Rules 2012, pp. 16–17. Although the sport is played almost exclusively by men, women are eligible to play in high school, college, and professional football. No woman has ever played in the NFL, but women have played in high school and college football games. In 2018, 1,100 of the 225,000 players in youth football were girls, and around 11% of the 5.5 million Americans who report playing tackle football are female according to the Sports and Fitness Industry Association. Women can also serve as officials; Sarah Thomas became the NFL's first female official in 2015.
Offensive unitThe role of the offensive unit is to advance the football down the field with the ultimate goal of scoring a . The offensive team must line up in a legal before they can snap the ball. An offensive formation is considered illegal if there are more than four players in the or fewer than five players numbered 50–79 on the offensive line.NCAA Rules 2011–2012, p. 15.NFL Rules 2012, pp. 21–24.NFHS Rules 2012, pp. 57–58. Players can line up temporarily in a position whose eligibility is different from what their number permits as long as they report the change immediately to the referee, who then informs the defensive team of the change.NFL Rules 2012, pp. 36, 40. Neither team's players, except the (C), 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. The main backfield positions are the (QB), halfback/tailback (HB/TB) and fullback (FB). The quarterback is the leader of the offense. Either the quarterback or a coach calls the plays. Quarterbacks typically inform the rest of the offense of the play in the 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. The primary role of the halfback, also known as the running back or 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 and are seldom used in passing situations. The (OL) consists of several players whose primary function is to members of the defensive line from tackling the ball carrier on running plays or sacking the quarterback on passing plays. The leader of the offensive line is the center, who is responsible for snapping the ball to the quarterback, blocking, and for making sure that the other linemen do their jobs during the play. On either side of the center are the guards (G), while (T) line up outside the guards. The principal receivers are the s (WR) and the s (TE). Wide receivers line up on or near the line of scrimmage, split outside the line. The main goal of the wide receiver is to catch passes thrown by the quarterback, but they may also function as decoys or as blockers during running plays. Tight ends line up outside the tackles and function both as receivers and as blockers.
Defensive unitThe role of the defense is to prevent the offense from scoring by the ball carrier or by forcing turnovers ( s or s). The (DL) consists of s (DE) and (DT). Defensive ends line up on the ends of the line, while defensive tackles line up inside, between the defensive ends. The primary responsibilities of defensive ends and defensive tackles are to stop running plays on the outside and inside, respectively, to pressure the quarterback on passing plays, and to occupy the line so that the s can break through. 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. The field, often called the secondary, consists of s (CB) and safeties (S). Safeties are themselves divided into free safeties (FS) and strong safeties (SS). Cornerbacks line up outside the defensive formation, typically opposite a receiver to be able to cover them. Safeties line up between the cornerbacks but farther back in the secondary. Safeties are the last line of defense and are responsible for stopping deep passing plays as well as running plays.
Special teams unitThe special teams unit is responsible for all kicking plays. The special teams unit of the team in control of the ball tries to execute field goal (FG) attempts, punts and kickoffs, while the opposing team's unit will aim to block or return them. Three positions are specific to the field goal and PAT (point-after-touchdown) unit: the (K or PK), holder (H) and (LS). The long snapper's job is to snap the football to the holder, who will catch and position it for the placekicker. There is not usually a holder on kickoffs, because the ball is kicked off a tee; however, a holder may be used in certain situations, such as if wind is preventing the ball from remaining upright on the tee. The player on the receiving team who catches the ball is known as the (KR). The positions specific to punt plays are the punter (P), long snapper, and . The long snapper snaps the football directly to the punter, who then drops and kicks it before it hits the ground. Gunners line up split outside the line and race down the field, aiming to tackle the (PR)—the player who catches the punt. Upbacks line up a short distance behind the line of scrimmage, providing additional protection to the punter.
ScoringIn football, the winner is the team that has scored more points at the end of the game. There are multiple ways to score in a football game. The touchdown (TD), worth six points, is the most valuable scoring play in American football. A touchdown is scored when a live ball is advanced into, caught in, or recovered in the opposing team's end zone. The scoring team then attempts a try or conversion, more commonly known as the point(s)-after-touchdown (PAT), which is a single scoring opportunity. A PAT is most commonly attempted from the two- or three-yard line, depending on the level of play. If a PAT is scored by a placekick or dropkick through the goal posts, it is worth one point, typically called the extra point. If it is scored by what would normally be a touchdown it is worth two points, typically called the . In general, the extra point is almost always successful while the two-point conversion is a much riskier play with a higher probability of failure; accordingly, extra point attempts are far more common than two-point conversion attempts. A field goal (FG), worth three points, is scored when the ball is placekicked or dropkicked through the uprights and over the crossbars of the defense's goalposts.NFL Rules 2012, pp. 57–59.NCAA Rules 2011–2012, pp. 79–80.NFHS Rules 2012, p. 66. After a PAT attempt or successful field goal, the scoring team must kick the ball off to the other team. A is scored when the ball carrier is tackled in their own end zone. Safeties are worth two points, which are awarded to the defense. In addition, the team that conceded the safety must kick the ball to the scoring team via a .NFL Rules 2012, p. 60
Field and equipmentFootball games are played on a rectangular that measures long and wide. Lines marked along the ends and sides of the field are known as the end lines and sidelines. Goal line (American football), Goal lines are marked inward from each end line.NFL Rules 2012, p. v, 1.NCAA Rules 2011–2012, pp. 18–19, 23–24.NFHS Rules 2012, pp. 11–12, 13, 28. Weighted pylons are placed the sidelines on the inside corner of the intersections with the goal lines and end lines. White markings on the field identify the distance from the end zone. Inbound lines, or , are short parallel lines that mark off increments. , which can run the width of the field, are marked every . A one-yard-wide line 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 distance from the closest goal line in yards are placed on both sides of the field every ten yards. are located at the center of the plane of the two end lines. The crossbar of these posts is above the ground, with vertical uprights at the end of the crossbar apart for professional and collegiate play, and apart for high school play. The uprights extend vertically on professional fields, a minimum of on college fields, and a minimum of on high school fields. Goal posts are padded at the base, and orange ribbons are normally placed at the tip of each upright as indicators of wind strength and direction.NFL Rules 2012, p. 2.NCAA Rules 2011–2012, p. 18.NFHS Rules 2012, p. 14. The itself is an oval ball, similar to the balls used in rugby or . At all levels of play, the football is inflated to and weighs ;NCAA Rules 2011–2012, p. 20.NFL Rules 2012, p. 3. beyond that, the exact dimensions vary slightly. In professional play the ball has a long axis of , a long circumference of , and a short circumference of . In college and high school play the ball has a long axis of , a long circumference of , and a short circumference of .
Duration and time stoppagesFootball 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.NFL Rules 2012, p. 14.NCAA Rules 2011–2012, p. 45. High school football games are 48 minutes in length with two halves of 24 minutes and four quarters of 12 minutes.NFHS Rules 2012, pp. 38–39. The two halves are separated by a half-time">halftime period, and the first and third quarters are followed by a short break.NFHS Rules 2012, p. 39. Before the game starts, the referee and each team's captain (sports)">captain meet at midfield for a coin toss. The visiting team can call either "heads" or "tails"; the winner of the toss chooses whether to receive or kick off the ball or which goal they wish to defend. They can defer their choice until the second half. Unless the winning team decides to defer, the losing team chooses the option the winning team did not select—to receive, kick, or select a goal to defend to begin the second half. Most teams choose to receive or defer, because choosing to kick the ball to start the game allows the other team to choose which goal to defend. Teams switch goals following the first and third quarters. If a down is in progress when a quarter ends, play continues until the down is completed.NFL Rules 2012, pp. 14–18NCAA Rules 2011–2012, pp. 47–53.NFHS Rules 2012, pp. 38–45 Games last longer than their defined length due to play stoppages—the average NFL game lasts slightly over three hours. 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 .NCAA Rules 2011–2012, pp. 16, 41. A separate infraction 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 25 seconds after certain administrative stoppages in play and to 40 seconds when play is proceeding without such stoppages.NCAA Rules 2011–2012, pp. 41, 46–47.NFHS Rules 2019.
Advancing the ball and downsThere are two main ways the offense can advance the ball: and passing. In a typical play, the center passes the ball backwards and between their 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. 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. A forward pass can be legally attempted only if the passer is behind the line of scrimmage; only one forward pass can be attempted per down. Like in rugby, players can also pass the ball backwards at any point during a play. In the NFL, a down also ends immediately if the runner's helmet comes off. 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 football is turned over to the defense. In most situations, if the offense reaches their fourth down they will the ball to the other team, which forces them to begin their drive from farther down the field; if they are in , they might attempt to score a field goal instead. A group of officials, the chain crew, keeps track of both the downs and the distance measurements. On television, a yellow line is electronically superimposed on the field to show the first down line to the viewing audience.
KickingThere 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,NFL Rules 2012, p. 50.NCAA Rules 2011–2012, p. 34.NFHS Rules 2012, p. 32. and free kicks.NFL Rules 2012, p. 6.NCAA Rules 2011–2012, p. 30.NFHS Rules 2012, p. 27. 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; the safety kick follows a safety.NFL Rules 2012, pp. 8–9.NFHS Rules 2012, pp. 31–32. 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 a tee; 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. The ball may be recovered by the kicking team once it has gone at least ten yards and has touched the ground or has been touched by any member of the receiving team.NFL Rules 2012, pp. 29–30.NCAA Rules 2011–2012, pp. 61–64.NFHS Rules 2012, pp. 15, 46, 52–53. The three types of scrimmage kicks are place kicks, drop kicks, and punts. Only place kicks and drop kicks can score points. The place kick is the standard method used to score points, because the pointy shape of the football makes it difficult to reliably drop kick. 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.NFL Rules 2012, pp. 33–34, 50–53.NCAA Rules 2011–2012, pp. 55–56, 63–64.NFHS Rules 2012, pp. 49, 53–54. The kicking team is prohibited from interfering with the receiver's opportunity to catch the ball. The receiving team has the option of signaling for a , which prohibits the defense from blocking into or tackling the receiver. The play ends as soon as the ball is caught and the ball may not be advanced.NFL Rules 2012, pp. 7, 54–55.NCAA Rules 2011–2012, pp. 30, 66–67.NFHS Rules 2012, pp. 27, 56.
Officials and foulsOfficials are responsible for enforcing game rules and monitoring the clock. All officials carry a 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 has been called. An official who spots multiple fouls will throw their hat as a secondary signal. The seven officials (of a standard seven-man crew; lower levels of play up to the college level use fewer officials) on the field are each tasked with a different set of responsibilities: * The referee is positioned behind and to the side of the offensive backs. The referee is charged with oversight and control of the game and is the authority on the score, the down number, and any rule interpretations in discussions among the other officials. The referee 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 referee positions themselves to the passing arm side of the quarterback. In most games, the referee is responsible for spotting the football prior to a play from scrimmage. * The umpire is positioned in the defensive backfield, except in the NFL, where the umpire is positioned lateral to the referee on the opposite side of the formation. The umpire watches play along the line of scrimmage to make sure that no more than 11 offensive players are on the field before the snap and that no offensive linemen are illegally downfield on pass plays. The umpire monitors 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 back judge is positioned deep in the defensive backfield, behind the umpire. 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 violation occurred. The back judge is also responsible for the play clock, the time between each play, when a visible play clock is not used. * The head linesman/down judge is positioned on one end of the line of scrimmage. The head linesman/down judge 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/down judge also rules on out-of-bounds calls that happen on their side of the field, oversees the chain crew and marks the forward progress of a runner when a play has been whistled dead. * The side judge is positioned twenty yards downfield of the head linesman. The side judge mainly duplicates the functions of the field judge. On field goal and extra point attempts, the side judge is positioned lateral to the umpire. * The line judge is positioned on the end of the line of scrimmage, opposite the head linesman. They supervise player substitutions, the line of scrimmage during punts, and game timing. The line judge 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. * The field judge is positioned twenty yards downfield from the line judge. The field judge monitors and controls the , are responsible for moving the chains. The chains, consisting of two large sticks with a 10-yard-long chain between them, are used to measure for a first down. The chain crew stays on the sidelines during the game, but if requested by the officials they will briefly bring the chains on to the field to measure. A typical chain crew will have at least three people—two members of the chain crew will hold either of the two sticks, while a third will hold the down marker. The down marker, a large stick with a dial on it, is flipped after each play to indicate the current down and is typically moved to the approximate spot of the ball. The chain crew system has been used for over 100 years and is considered to be an accurate measure of distance, rarely subject to criticism from either side.
Safety and brain healthFootball is a full-contact sport, and injuries are relatively common. Most injuries occur during training sessions, particularly ones that involve contact between players. To try to prevent injuries, players are required to wear a set of equipment. At a minimum players must wear a and a set of , but individual leagues may require additional padding such as thigh pads and guards, knee pads, chest protectors, and s.NFL Rules 2012, pp. 24–27.NCAA Rules 2011–2012, p. 22.NFHS Rules 2012, pp. 17–19. Most injuries occur in the lower extremities, particularly in the knee, but a significant number also affect the upper extremities. The most common types of injuries are strains, s, s, fractures, , and s. Repeated (and possibly sub-concussive head impacts) can increase a person's risk in later life for CTE ( ) and mental health issues such as , , and . Concussions are often caused by helmet-to-helmet or upper-body contact between opposing players, although helmets have prevented more serious injuries such as s. Various programs are aiming to reduce concussions by reducing the frequency of helmet-to-helmet hits; 's "Heads Up Football" program aims to reduce concussions in youth football by teaching coaches and players about the signs of a concussion, the proper way to wear football equipment and ensure it fits, and proper tackling methods that avoid helmet-to-helmet contact. However, a study in the ''Orthopaedic Journal of Sports Medicine'' found that Heads Up Football was ineffective; the same study noted that more extensive reforms implemented by and its member teams were effective in significantly reducing concussion rates. A 2018 study performed by the VA Boston Healthcare System and the found that tackle football before age 12 was correlated with earlier onset of symptoms of CTE, but not with symptom severity. More specifically, each year a player played tackle football under age 12 predicted earlier onset of cognitive, behavioral, and mood problems by an average of two and a half years.Age of First Exposure to Tackle Football and Chronic Traumatic Encephalopathy
Leagues and tournamentsThe (NFL) and the (NCAA) are the most popular football leagues in the United States. The National Football League was founded in 1920 and has since become the largest and most popular sport in the United States. The NFL has the highest average attendance of any sporting league in the world, with an average attendance of 66,960 during the 2011 NFL season. The NFL championship game is called the Super Bowl, and is among the biggest events in club sports worldwide. It is played between the champions of the (NFC) and the (AFC), and its winner is awarded the . is the third-most popular sport in the United States, behind professional baseball and professional football. The NCAA, the largest collegiate organization, is divided into three Divisions: , Division II and Division III. Division I football is further divided into two subdivisions: the (FBS) and the (FCS). The champions of each level of play are determined through NCAA-sanctioned playoff systems; while the champion of Division I-FBS was historically determined by various polls and ranking systems, the subdivision adopted a four-team playoff system in 2014. is the most popular sport in the United States played by boys; over 1.1 million boys participated in the sport from 2007 to 2008 according to a survey by the (NFHS). The NFHS is the largest organization for high school football, with member associations in all as well as the . USA Football is the governing body for youth and amateur football, and Pop Warner Little Scholars is the largest organization for youth football.
Other professional leaguesSeveral professional football leagues have been formed outside the auspices of the NFL. One such league, the XFL, is currently operating as of 2020.
Rival leaguesThe most successful league to directly compete with the NFL was the American Football League (AFL), which existed from 1960 to 1969. The AFL became a significant rival in before signing a five-year, US$36 million television deal with NBC. AFL teams began signing NFL players to contracts, and the league's popularity grew to challenge that of the NFL. The two leagues merged in the 1970 season, and all the AFL teams joined the NFL. An earlier league, the (AAFC), was in play from 1946 to 1949. After it had dissolved, two AAFC teams, the and the , became members of the NFL; another member, the joined the league, but folded after just a year in the NFL. Other attempts to start rival leagues since the AFL merged with the NFL in 1970 have been far less successful, as professional football salaries and the NFL's television contracts began to escalate out of the reach of competitors and the NFL covered more of the larger cities. The (WFL) played for two seasons, in 1974 and 1975, but faced such severe monetary issues it 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 the season could be completed. The (USFL) operated for three seasons from 1983 to 1985. Originally not intended as a rival league, the entry of owners who sought marquee talent and to challenge the NFL led to an escalation in salaries and ensuing financial losses. A subsequent US$1.5 billion antitrust lawsuit against the NFL was successful in court, but the league was awarded only $1 in damages, which was automatically tripled to $3 under antitrust law.
Complementary national leaguesThe original XFL was created in 2001 by and lasted for only one season. Despite television contracts with NBC and , and high expectations, the XFL suffered from low quality of play and poor reception for its use of tawdry professional wrestling gimmicks, which caused initially high ratings and attendance to collapse. (The current XFL, also owned by McMahon, does not include those gimmicks.) The United Football League (UFL) began in but folded after suspending its 2012 season amid declining interest and lack of major television coverage. The lasted less than one season, unable to keep investors.
International playAmerican football leagues exist throughout the world, but the game has yet to achieve the international success and popularity of and . It is not an , but it was a at the . At the international level, Canada, Mexico, and Japan are considered to be second-tier, while Austria, Germany, and France would rank among a third tier. These countries rank far below the United States, which is dominant at the international level. , the of the NFL, operated from 1991 to 1992 and then from 1995 to 2007. At the time of its closure, NFL Europa had five teams based in Germany and one in the Netherlands. In Germany, the (GFL) has 16 teams and has operated for over 40 seasons, with the league's championship game, the , closing out each season. The league operates in a structure with (GFL2), which also has 16 teams. The functions as a continental championship for Europe. The competition is contested between the top six European teams. The also operated several teams within NFL Europe during the League's tenure, the resulting rise in popularity of the sport brought the NFL back to the country in 2007 where they now hold the in , currently consisting of four regular season games. The continuing interest and growth in both the sport and the series has led to the possible formation of a potential NFL franchise in London An American football league system already exists within the UK, the BAFANL, which has ran under various guises since 1983. It currently has 70 teams operating across the tiers of contact football in which teams aim to earn promotion to the Division above, with the Premier Division teams competing to win the , the annual British Football Bowl game that has been played since 1985. In 2007 the was formed. From 2008, the BUAFL was officially associated with the (NFL), through its partner organisation NFL UK. In 2012, BUAFL's league and teams were absorbed into BUCS after American football became an official BUCS sport. Over the period 2007 to 2014, the BUAFL grew from 42 teams and 2,460 participants to 75 teams and over 4,100 people involved. American football federations are present in , , , and ; a total of 64 national football federations exist as of July 2012. The (IFAF), an international governing body composed of continental federations, runs tournaments such as the , 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. The IFAF has received provisional recognition from the (IOC). Several major obstacles hinder the IFAF goal of achieving status as an Olympic sport. These include 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. American football also has an issue with a lack of global visibility. , the CEO of , noted that "American football is recognized globally as a sport, but it's not played globally." To solve these concerns, major effort has been put into promoting , a modified version of American football, at the international level.
Popularity and cultural impact
United States"Baseball is still called the national pastime, but football is by far the more popular sport in American society", according to ESPN.com's Sean McAdam. In a 2014 poll conducted by , professional football ranked as the most popular sport, and college football ranked third behind only professional football and baseball; 46% of participants ranked some form of the game as their favorite sport. Professional football has ranked as the most popular sport in the poll since 1985, when it surpassed baseball for the first time. Professional football is most popular among those who live in the eastern United States and rural areas, while college football is most popular in the southern United States and among people with graduate and post-graduate degrees. Football is also the most-played sport by high school and college athletes in the United States. In a 2012 study, the NCAA estimated there were around 1.1 million high school football players and nearly 70,000 college football players in the United States; in comparison, the second-most played sport, basketball, had around 1 million participants in high school and 34,000 in college. The Super Bowl is the most popular single-day sporting event in the United States, and is among the biggest club sporting events in the world in terms of TV viewership. The NFL makes approximately $10 billion annually. Super Bowl games account for seven of the top eight most-watched broadcasts in American history; , played on February 1, 2015, was watched by a record 114.4 million Americans. American football also plays a significant role in American culture. The day on which the Super Bowl is held is considered a ''de facto'' national holiday, and in parts of the country like , the sport has been compared to a religion. Football is also linked to other holidays; New Year's Day is traditionally the date for several college football bowl games, including the Rose Bowl. However, if New Year's Day is on a Sunday, the bowl games are moved to another date so as not to conflict with the typical NFL Sunday schedule. is an American tradition, hosting many high school, college, and professional games. Steve Deace of '' '' wrote that Americans are passionate about football "because it embodies everything we love about