<Tr> <Th> Country or region </Th> <Td> Worldwide (most prominent in North America, Europe, and Japan) </Td> </Tr> <Tr> <Th> Olympic </Th> <Td> No (demonstrated at the 1932 Summer Olympics) </Td> </Tr> <P> American football, referred to as football in the United States and Canada, and also known as gridiron football or simply gridiron, is a sport played by two teams of eleven players on a rectangular field with goalposts at each end . The offense, which is the team controlling the oval - shaped football, attempts to advance down the field by running with or passing the ball, while the defense, which is the team without control of the ball, aims to stop the offense's advance and aims to take control of the ball for themselves . The offense must advance at least ten yards in four downs, or plays, and otherwise they turn over the football to the defense; if the offense succeeds in advancing ten yards or more, they are given a new set of four downs . Points are primarily scored by advancing the ball into the opposing team's end zone for a touchdown or kicking the ball through the opponent's goalposts for a field goal . The team with the most points at the end of a game wins . </P> <P> American football evolved in the United States, originating from the sports of association football and rugby football . The first game of American football was played on November 6, 1869, between two college teams, Rutgers and Princeton, under rules based on the association football rules of the time . During the latter half of the 1870s, colleges playing association football switched to the Rugby Union code, which allowed carrying the ball . A set of rule changes drawn up from 1880 onward by Walter Camp, the "Father of American Football", established the snap, eleven - player teams, and the concept of downs; later rule changes legalized the forward pass, created the neutral zone, and specified the size and shape of the football . </P>

What is the number of players on a nfl team