American football, known in the United States simply as football and sometimes as
gridiron outside the United States and Canada, is a sport played between two teams of
eleven. The objective of the game is to score points by advancing the ball into the
opposing team's end zone. The ball can be advanced by running with it or throwing it to a
teammate. Points can be scored by carrying the ball over the opponent's goal line,
catching a pass thrown over that goal line, kicking the ball through the opponent's goal
posts or tackling an opposing ball carrier in his own end zone.
American football is closely related to Canadian football but with some differences in
rules and in the configuration of the field,  in particular the presence of eleven players
on the field rather than twelve and the allowance of four downs per possession rather than
just three. The larger number of downs results in more offensive rushing in American
football than in the Canadian game.
In the United States, the major forms are high school football, college football and
professional football, which are essentially similar but feature slightly different rules.
High school football is governed in the U.S. by the National Federation of State High
School Associations. College football is governed in the U.S. by two bodies: the National
Collegiate Athletic Association and the National Association of Intercollegiate Athletics.
The major league for professional football is the National Football League. Over the
years, there have been other notable professional football leagues, including the All
America Football Conference during the 1940s, the American Football League during the
1960s, the United States Football League during the 1980s, and the currently active
United Football League.
The sport is also played outside the United States. National and collegiate leagues exist in
the United Kingdom, Germany, Italy, Switzerland, Finland, Sweden, Japan, Mexico,
Israel, Spain, Austria, and several other European, American, Asian and Pacific Island
nations. The International Federation of American Football acts as an international
governing body for the sport on five continents, but the organization has little standing in
the United States compared to the aforementioned bodies and leagues.
o2.1 Field and players
o2.2 Start of halves
o2.3 Game duration
o2.4 Advancing the ball
o2.5 Change of possession
o2.7 Kickoffs and free kicks