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Basic Rules
The American game is played by two opposing teams of eleven. The football field is level, measures 100 by 53 1/3 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.
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