football, American Contact sport played mainly in the USA. It is second in popularity only to baseball. It is played by two teams of 11 people on a field 100 × 53yd (91.5 × 49m). The field is marked off by latitudinal stripes every 5yd (4.6m), and is flanked on each end by an end zone, 10yd (9.1m) long. At each end of the end zone are H-shaped goal posts. An inflated leather, spheroid ball is used, with the object of moving the ball – by the ground or air – across the opponent's goal line. A game consists of two halves, each having two 15-minute quarters. Each half starts with a kick-off, and after the receiving team has run back the ball, it must advance 10yd in four attempts (downs) or turn the ball over to the opponents. The defending team must stop the ball carrier by pushing him out of bounds or by bringing him to the ground (tackling). The ball is usually turned over by punting (kicking) on the last down or attempting a field goal. If a player fumbles and loses possession of the ball during the series of downs, the opposition takes over the ball. Scoring can occur in four ways: a touchdown (crossing the opponent's goal line) scores six points; a conversion (kicking the ball through the uprights of the goal post after a touchdown) scores one; a field goal (kicking the ball through the uprights in normal play) scores three; and a safety (downing the ball carrier behind his own goal line) scores two points. Except for certain instances in professional football, games can end in a tie. Substitutions are freely allowed. Football has its roots in England and has similarities to rugby and soccer. Professional football began in the 19th century. The winners of the two US professional leagues, the American Football Conference (AFL) and the National Football Conference (NFL), play off each January for the Super Bowl.