American football (known simply as football in the United States, and gridiron in some other countries) is a sport played by two teams of eleven players on a rectangular field 120 yards long by 53.3 yards wide with goalposts at each end. The offense attempts to advance an oval ball down the field by running with or passing it. They must advance it at least ten yards in four downs to receive a new set of four downs and continue the drive; if not, they turn over the ball to the opposing team. Points are scored by advancing the ball into the end zone for a touchdown, kicking the ball through the opponent's goalposts for a field goal or by the defense tackling the ball carrier in the offense's end zone for a safety.
American football and its rules evolved from other codes of football, rugby and association (soccer). The first game was played on November 6, 1869. A set of rule changes drawn up from 1880 onward by Walter Camp established the snap, eleven-player teams and downs. Later rule changes legalized theforward pass, created the neutral zone and specified the width of the football. It is one of the two main forms of gridiron football, the other being Canadian football.