The football field measures 91.4m by 48.8m and is marked off by latitudinal stripes every 4.57m. At either end there is and endzone which measures 9.14m across. In the center of each endzone stand goal posts 6.1m high, with a crossbar 3.05m above the ground and with the uprights on either end 7.2m apart.
The object of the game is to move 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 sideways), but the team must gain 10 yd (9 m) in four plays (downs) or they lose possession of the ball. The defending team tries to stop the man with the ball by tackling 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 for offense and defense differ, most teams have offensive and defensive squads that alternate on the field as possession of the ball changes. All professional teams also have special teams for various game situations (e.g., offense, defense, kickoffs, and punt returns). The offensive team usually comprises a quarterback, a fullback, two halfbacks, 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 four quarters lasing 15 minutes each. 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 tackling 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 scoring a touchdown. The conversion is scored 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).
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.