What were allowed in the English Football League for the first time in the 1965-66 season?

Substitutions during matches in the English Football League were first permitted in the 1965–66 season. During the first two seasons after the law was introduced, each side was permitted only one substitution during a game. Moreover, the substitute could only replace an injured player. From the 1967–68 season, this rule was relaxed to allow substitutions for tactical reasons.

In association football, a substitute is a player who is brought on to the pitch during a match in exchange for an existing player. Substitutions are generally made to replace a player who has become tired or injured, or who is performing poorly, or for tactical reasons (such as bringing a striker on in place of a defender). Unlike some sports (such as American football or ice hockey), a player who has been substituted during a match may take no further part in it.

Most competitions only allow each team to make a maximum of three substitutions during a game, although more substitutions are often permitted in non-competitive fixtures such as friendlies.