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Gridiron football, also known as North American football or, in North America, simply football, is a football sport primarily played in the United States and Canada. American football, which uses 11-player teams, is the form played in the United States and the best known form of gridiron football worldwide, while Canadian football, featuring 12-player teams, predominates in Canada. Other derivative varieties include indoor football, football for smaller teams (most commonly eight players), and informal games such as touch and flag football. Football is played at professional, collegiate, semi-professional, and amateur levels.