The Burnside rules were a set of rules that transformed Canadian football from a rugby-style game to the gridiron-style game it has remained ever since. Named after Thrift Burnside, captain of the University of Toronto football team (although he did not originate them), and first adopted by the Ontario Rugby Football Union in 1903, the rules introduced sweeping changes to the way football was played. The rules included:
- a reduction from 15 to 12 players per side
- a reduction from 8 to 6 men allowed on the line of scrimmage when the ball was put into play
- the "snap-back" system in which the ball was heeled backward from the line of scrimmage by the centre
- a requirement for a team to make ten yards in three successive downs or lose possession of the ball
Although similar to American football rules already in place at the time, which had been developed by Walter Camp in the 1880s, Burnside rules had many differences and evolved separately. Although these rules are standard today, at the time they were considered radical. Other teams outside the Ontario Rugby Football Union refused to adopt them until 1905.