sentence: 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 yard in three successive down 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 .