The one-platoon system, also known as iron man football, is a platoon system in American football where players play on both offense and defense. It was the result of smaller roster sizes in the early days of the game and rules that limited player substitutions, rules that are also standard procedure in many other sports but were eliminated in the 1940s as free substitution was legalized. The alternative system is the two-platoon system (or simply the platoon system), which uses separate offensive and defensive units (three platoons if special teams is also counted).