"Up to a maximum of three substitutions may be employed in any match played in an official tournament organized under the auspices of FIFA, the confederations, or the member associations," according to the Laws of the Game. In addition, up to six substitutions may be employed in national A team matches.
Substitutions are useful in several situations: when a player is injured (or if he decides not to leave the field); when a coach wants to make a change; or as a means for introducing some youth into the game (especially in developmental tournaments). Substitutes often come off the bench and are introduced by the referee at any time during the game. The number of substitutes allowed is usually based on the size of the pitch but may also be based on the number of players per side, either explicitly or through the use of 11 vs. 11 football.
In England, Scotland and Wales substitutes are usually recruited from the back of the squad. This is known as the "three substitutes rule". Each club is allowed a specific number of replacements, which can vary between two and five. If a player is sent off or receives a second yellow card, then another player will take his place. A red card immediately ends the replacement's career and requires that they be replaced by a new player brought onto the field by one of the teams.
In South America, Central America and Mexico substitutes are usually chosen from among the younger players in the squad.
There are nine substitutions. In every match played in an official tournament organized by FIFA, the confederations, or the member associations, a maximum of nine substitutions may be utilized. The competition regulations must specify how many substitutes may be nominated, up to a maximum of nine. A player is replaced during extra time; however, he cannot be replaced again after it has started.
In club games and tournaments where the number of substitutions is not limited, the coach can choose when to use them. A team is said to have used its full quota of substitutions when all its players are replaced.
The number of substitutions per game is usually one or two. Larger numbers are used in some competitions such as the UEFA Champions League and FIFA Club World Cup. These numbers are often due to the greater physical strength of some players compared to others, or because a coach wants to keep his best players on the field as long as possible.
There is no limit on the number of times a player can be substituted during a single match. However, most coaches prefer not to use more than two subsets of players in order to avoid having too many changes on the field at one time. This is especially important when you consider that each substitution takes approximately six minutes of playtime away from the end of the first half and the start of the second.
Except for men's and women's competitions involving the first teams of clubs in the top division or senior "A," the number of substitutes, up to a maximum of five, that may be used in any match played in an official competition will be determined by FIFA, the confederation, or the national football association...
In most countries, substitutions are made by the team captain when his side is losing (or drawing), with the aim of changing the game in their favor. It is common practice at all levels of football around the world, from amateur to professional, male or female, to give every player on the field an opportunity to play.
However, some leagues have a limit on the total number of substitutions that can be made by both sides in a single match. For example, the English Premier League allows two changes to the lineup per side, with a further four added if the game is going into extra time. A club cannot use more than three substitutes in any match.
The substitution system was originally designed to allow players to come off the bench and provide inspiration or experience for the younger members of the squad. Substitutes are also used as a way to control the amount of physical contact between opponents and prevent injuries.
However, over the years, managers have found ways to manipulate the system in their own favor.
Law 3: The Players—Temporary Change Each side will be allowed a maximum of five replacements. To minimize play disturbance, each side will have a maximum of three opportunities to make substitutes throughout the game; substitutions may also be performed at half-time. If a player is replaced during play, the replacement shall come from the sideline nearest the foul.
Examples: In regulation time, a team scores a free throw with 0.4 seconds left and loses by one point (49-48). That team could replace its player after making its fifth substitution (or fourth if it used its first two substituting for technical reasons). In overtime, a team makes its sixth substitution.
The example assumes the fouled player was on the court when the shot went up. If he wasn't, the replacement would come from the bench.
A player who comes off the floor during play due to injury will not be allowed to return to the game. If the injured player needs to be taken out of the game, a temporary replacement will be brought in according to Law 3. This replacement can be any player, including himself, as long as he's not still on the roster of his previous team. For example, if a player leaves the game because of an injury to his knee, that player cannot return until his knee has healed enough for him to play without pain.