The American game is played by two opposing teams of eleven. The football field is level, measures 100 by 531/3 yd (91.4 by 48.8 m), is marked off by latitudinal stripes every 5 yd (4.57 m) and has at each end an end zone 10 yd (9.14 m) deep. In the center of each end zone stand goal posts not exceeding 20 ft (6.10 m) in height, with a crossbar 10 ft (3.05 m) from the ground and with the uprights on either end 24 ft (73.2 m) apart.
Play is directed toward moving the ball across the opponent's goal line, thereby scoring a touchdown, worth six points. In advancing the ball a team may run with it or pass it (forward or laterally), but the team must gain 10 yd (9 m) in four plays (downs) or yield possession of the ball to the opponent. The defending team tries to stop the ball carrier by tackling him, i.e., forcing him to the ground—thus causing the team with the ball to use up one of its downs. The defending team can gain possession of the ball before the end of four downs by recovering a dropped ball (fumble), or by intercepting a pass.
Because strategies and skills required on offense and on defense differ, most organized football clubs have offensive and defensive squads that alternate on the field as possession of the ball changes. All professional and most collegiate teams employ special teams for various game situations (e.g., offense, defense, kickoffs, and punt returns) and coaches who specialize in various aspects of the game. The offensive team traditionally comprises a quarterback (the field leader), a fullback, two halfbacks, and seven linemen—a center, two guards, two tackles, and two ends. A typical defensive unit has two tackles and two ends, who play on the line, as well as three linebackers, two cornerbacks, and two safeties.
The game is divided into two halves, each consisting of two quarters, periods of 15 minutes playing time. At the end of each of the first three quarters, the teams exchange goals. Each half begins with a kickoff, which also initiates play after every score (except a safety). In addition to the touchdown, points are scored by kicking the ball (which is held on the ground by a teammate of the kicker) over the crossbar between the goal posts (a field goal), for three points; and by downing a player in possession of the ball behind his own goal line (a safety), for two points. Additional points, known as conversions, may be scored after the scoring of a touchdown. In professional play the conversion is earned by kicking the ball over the crossbar of the goal post (worth one point) or by running or passing the ball over the goal line from 2 yd (1.83 m) away (worth two points). In amateur (high school and college) football, the conversion play is begun 3 yd (2.74 m) from the end zone.
When a team is not likely to gain 10 yards in four downs, it often kicks, or punts, the ball downfield, usually on the fourth down. After each down, before resuming play, the opposing teams face each other along an imaginary line, called the line of scrimmage, determined by the position of the ball relative to the goals. Among standard offensive formations, the basic T formation (a balanced line with the quarterback behind the center and the other backs behind the quarterback) is, with modern variations, the most popular in both amateur and professional football. Blocking and tackling make football one of the most rugged sports played; thus players wear heavy protective gear.
Five officials—the referee, umpire, field judge, linesman, and electric clock operator—control a game, and penalties, chiefly in the form of moving the ball away from a team's object goal, are meted out for violations of the rules. Rules concerning the field, scoring, playing time, downs, scrimmage, substitution, officials, and equipment have undergone numerous changes, generally to make the game safer.
The first intercollegiate football match in America (actually a 50-person soccer game) was played (1869) at New Brunswick, N.J. The Intercollegiate (Soccer) Football Association, composed of Columbia, Princeton, Rutgers, and Yale, was created (1873) to standardize rules. Harvard, meanwhile, refused to join the group and, looking for other opponents, accepted a challenge from McGill Univ. of Montreal to play a series of games (1874–75) under Rugby rules. The Rugby-type game soon caught on at the other schools also, and within a decade the distinctive game of American football evolved.
Since the late 19th cent. football has enjoyed tremendous popularity as a collegiate sport. In 1902 the first Rose Bowl game was played at Pasadena, Calif., and that postseason tournament has been conducted annually since 1916. Other annual, postseason, collegiate games include the Sugar, Orange, Sun, and Cotton bowls. In 1996 a national system to pick bowl opponents so as to determine a national champion was introduced. Selection of All-America teams, begun (1889) by Walter Camp and Caspar Whitney, has also contributed to football's popularity. The Heisman trophy, originated in 1935, is awarded annually to the nation's outstanding college football player.
Most collegiate teams play in athletic conferences. Among the best-known are the Ivy League, Big Ten, Atlantic Coast, Southeastern, and Pacific 10 conferences. Famous collegiate rivalries include Army–Navy and Yale–Harvard. With an atmosphere enhanced by bands and cheering sections, football is not only the most popular collegiate sport of the fall season but also a big business.
Revenues from football often finance other sports at a college, and some games are played before crowds of 100,000 people in university-owned stadiums. Despite the strict amateur code of the National Collegiate Athletic Association and its member conferences, illegal subsidization of football players is a recurrent issue. Football also is extremely popular in U.S. high schools. Six-man football and touch football, both usually played for recreation, are other forms of the amateur game.
Although professional football was played as early as 1895 in Pennsylvania, it was not until 1920 that national organization began, with the formation of the American Professional Football Association at Canton, Ohio. Originally consisting of five teams, the association evolved and in 1922 was renamed the National Football League (NFL). The professional game received a tremendous boost when Red Grange, a star halfback at the Univ. of Illinois, signed a professional contract (1925) with the Chicago Bears. Other college stars soon followed, and the public began to show interest in NFL teams.
In the period immediately following World War II professional football's popularity grew tremendously. A new league, the All-America Conference (established 1944), competed with the NFL until the two groups merged (1949). The American Football League (AFL; formed 1959) competed with the NFL during the early 1960s; the first Super Bowl championship game was held in 1967 between the NFL and AFL champions.
Four years later the two leagues were merged into the present NFL, which now comprises two conferences (the National Football Conference, or NFC, and the American Football Conference, or AFC) totaling 32 teams. The NFL season includes 16 regular games, after which the winners of the three divisions in each conference, along with two "wild card" teams (the teams with the next-best record in each conference) play a four-round playoff culminating in the Super Bowl.
See Official National Football League Record & Fact Book (annual); NCAA, Football: The Official Football Records Book (annual); C. Carter and D. Sloan, The Sporting News Pro Football Guide (annual); B. Carroll et al., ed., Total Football II: The Official Encyclopedia of the National Football League (1999); M. MacCambridge, America's Game: The Epic Story of How Pro Football Captured a Nation (2004).