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.
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 physically difficult 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 disqualification.
Gridiron football. Players can be changed during any dead ball situation. 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. The one-platoon system was largely abolished in the 1940s but aspects of it remained in force in college football until the 1960s, at which time total free substitution was implemented.)
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 if a team ices the puck they may not make any changes until after the ensuing faceoff. Almost all professional hockey teams use up to four lines of forwards and three pairings of defensemen in rotation during any given game.
Association football: In association football, only a limited of substitutions are allowed (usually three, per IFAB rules). 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. A fourth substitution during extra time is being considered by FIFA.
Baseball and softball: In most leagues, 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). If a substituted player reenters the game, the umpire must determine who is the substituted player, and that player is ejected from the game. 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, and some recreational leagues allow unlimited substitution, with the caveat that players who return to the game must return to the same place in the batting order as they were before.
Volleyball: Under FIVB rules, which are followed by almost all leagues worldwide, teams are limited to six substitutions per set, which can only take place in dead-ball situations. The NCAA follows this substitution rule in the men's National Collegiate division (Division I/II), but allows 12 substitutions per set in Division III men's play and 15 per set in women's play for all divisions. Substitutions involving the libero, a specialist player who can only play in the back row, are not counted against the limit under FIVB rules, but the libero can only be replaced by the player whom he or she replaced (with exceptions in case of injury). The NCAA rules on the libero follow FIVB rules except that in NCAA women's play, the libero is allowed to serve in the same spot in the rotation as the player she replaced.
Australian rules football: The most notable sport whose substitution rules have aspects of both free and limited substitution is Australian rules football. In the top-level Australian Football League, each team has a playing squad of 22, of whom 18 are on the field and four are so-called "interchanges". At all levels of the game, players may be interchanged at any time, including during gameplay, but must enter and exit the field through the interchange area, a 15-metre stretch of the boundary line between the teams' benches (with exceptions in case of injury). In the AFL, each team is limited to 90 substitutions per match. Other leagues have different limits on the number of interchanges and the number of substitutions allowed.