Portal:American football/Intro

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American football, known in the United States and Canada simply as football and often internationally as a type of gridiron football, as against association football, is a competitive sport, most often contested at any given point between two teams comprising 11 players each. A team attempts to advance a ball, roughly a prolate spheroid, by carrying (rushing), throwing (passing), or, under specific circumstances, kicking the ball, on a field composed of grass or artificial turf, intending eventually to advance the ball into an opponent's end zone–located 100 yards (91.44 metres) from a team's own end zone–prior to the exhaustion of a team's series of three or four downs, for a touchdown (worth six points) or a field goal (worth three points); the team to have scored more points upon expiration the time alloted for the game, 60 minutes–divided into four equal quarters–during a professional event, is the victor.

The offensive unit of one team, featuring a quarterback to whom the ball is snapped who directs the play of his team, competes against an opponent's defensive unit; should the offense fail to advance at least 10 yards beyond the location (original line of scrimmage) at which it receives possession, it may surrender the ball to the opposing team, the offense of which then takes the ball, either by exhausting its supply of downs or by kicking the ball away (punting). Possession may also transfer should, in the course of active play, an offensive player drop (fumble) or throw (interception) the ball to a defensive player.

Various activities–both relative to activities during play that inappropriately provide a competitive advantage or are inordinately dangerous and to unsportsmanlike conduct–are proscribed by rule and are punished with the assessment of penalties by a referee, who heads a team, at the professional and collegiate levels, of seven officials. Offenses are dealt with in a variety of fashions, but most often with an assessment of yardage to the team aggrieved or the nullification of a play during which an offense occurred.

Several variants of the game exist, including arena football, indoor football, and flag football. Both Canadian football and American football are derived from rugby football, and while they have some shared origins, there are significant differences.