Free substitution

Free substitution is a rule in some sports that allows players to enter and leave the game for other players many times during the course of the game; and for coaches to bring in and take out players an unlimited number of times.

Sports that allow free substitution

• Basketball. Basketball allows unlimited substitution of players at dead balls, such as full times-out, out-of-bounds turnovers and fouls. Players are allowed to go out and come in again many times, unless they are disqualified or ejected. Players are often substituted for, since it is almost physically impossible to play an entire basketball game (40 minutes in most competitions, 48 in the NBA). Substitutions are also made if a player is getting too many personal fouls and is in danger of disqualifying himself.
• Gridiron football. Players can be substituted for between every football play. Most college and pro football teams use 11 completely different players on offense and defense. Often, substitution is made depending on the play being run; for example, a team might bring in a fullback or tailback for a rushing play, or several wide receivers for a passing play. There are also major substitutions made for special teams plays such as punting or kicking a field goal. (Historically, this was not the case; gridiron-based codes originally had a one-platoon system in place that required all players to play all phases of the game. One-platoon was abolished in the 1940s.)
• Ice hockey. Players can be substituted for at any point in the game, even when the puck is in play. The only exception is that in the NHL, if a team ices the puck they may not make any changes until after the ensuing faceoff. Most pro hockey teams use up to four lines of forwards and three pairings of defensemen in rotation during any given game.

Sports that do not allow free substitution

• Association football: In association football, only a limited of substitutions are allowed. Players may only be substituted during a stoppage in play. Players who leave the game may not return, and if a team runs out of substitutions and loses a player, they must play the rest of the game short handed. If a player who leaves the game reenters, that player is assessed a red card and is ejected.
• Baseball: As with association football, a player who is substituted for may not return to the game. Baseball does not have a cap on the number of substitutions that can be made, although in practice they may not make more substitutions than the number of substitutes they have. A player inherits the place in the batting order of the player for whom he is substituted, unless multiple people are substituted for at the same time (a double switch). Many youth baseball leagues, such as Little League, have a modified version of this rule, in which a player may return to the game one time after being replaced. If a substituted player reenters the game, the umpire must determine who is the substituted player, and that player is ejected from the game.
• Rugby football: Both rugby league and rugby union follow a similar procedure to association football. Limited exceptions to this rule exist in both codes, most notably the blood replacement.