The two-platoon system is a tactic in American football enabled by rules allowing unlimited substitution adopted during the 1940s. The "two platoons", offense and defense, are an integral part of the game today.
Although professional football has uninterruptedly retained the two-platoon system since 1949, in 1953 the NCAA took the collegiate game back to the one-platoon system with new limited substitution rules, changes made ostensibly for financial and competitive reasons. These rules remained in place until a return to unlimited substitution was made for the 1965 season.