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American football is played on a field of rectangular shape 120 yards long and 53.5 yards wide. The field is bounded on each end by an end line marked in white lime across the entire width of the field. The ten-yard areas at each end of the field are called end zones; it is into this area that a team must carry or pass the ball to score a touchdown. At the back of each end zone is a set of two vertical parallel uprights extending at least 20 feet up from the ground and connected by a horizontal crossbar ten feet high.

During the game, the teams are designated as either the offensive team--that is, the team with the ball--and the defensive team--the team defending a goal line against the offense. Each side has eleven men on the field. The object of the game is to score a greater number of points than the opposition. Six points are scored for a touchdown, which is made when an offensive player carries the ball into the end zone of the opposition.

After a team has scored a touchdown, it is permitted to try for a conversion. The team may kick the ball through the uprights for one point or carry the ball into the end zone from the three-yard line for two points. Another way of scoring is to kick a field goal through the uprights for three points. Finally, two points are scored for the defensive team for a safety, which occurs when an offensive team ends a play in possession of the ball behind its own goal line.

A game of football is played in four 15-minute quarters. Play is supervised by a group of impartial officials of varying number. In order to keep the game a sportsmanlike contest, penalties are imposed by officials for violations of the numerous rules of the game.

To start the game, the defense kicks the ball to the offense, which may run the ball back as far as it can. The defense stops the advancement of the ball by bringing the offensive player with the ball to the ground, or tackling him. The offensive team has four chances to advance the ball ten yards, after which the team gets another four chances. If the offense is unable to advance ten yards in four chances, the defense gets the ball. After the third chance, or down, the offense may decide to drop kick, or punt, the ball to the defense in order to push the defense farther back on the field. The offense may run or pass the ball, and the defense may recover lost balls or intercept passes, at which time the defense becomes the offense.