American football evolved in the United States, originating from the sport of rugby football. The first game of American football was played on November 6, 1869, between two college teams, Rutgers and Princeton, under rules resembling a mix of rugby and soccer. 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.
American football as a whole is the most popular sport in the United States; professional football and college football are the most popular forms of the game, with the other major levels being high school and youth football. As of 2012, nearly 1.1 million high school athletes and 70,000 college athletes play the sport in the United States annually. The National Football League, the most popular American football league, has the highest average attendance of any sports league in the world; its championship game, the Super Bowl, ranks among the most-watched club sporting events in the world, and the league has an annual revenue of around US$10 billion.