In ice hockey, a line is a group of forwards that play in a group, or "shift", during a game.
A complete forward line consists of a left wing, a center, and a right wing, while a pair of defensemen who play together are called "partners." Typically, an NHL team dresses twelve forwards along four lines and three pairs of defensemen, though some teams elect to dress a seventh defenseman, or a thirteenth forward. In ice hockey, players are substituted "on the fly," meaning a substitution can occur even in the middle of play as long as proper protocol is followed (under typical ice hockey rules, the substituting player cannot enter the ice until the substituted player is within a short distance of the bench and not actively playing the puck); substitutions can still be made during stoppages. Usually, coordinated groups of players (called linemates) are substituted simultaneously in what are called line changes. Linemates may change throughout the game at the coach's say.
Ice hockey is one of only a handful of sports (gridiron football and basketball being the two most prominent others) that allows for unlimited free substitution and uses a system of multiple sets of players for different situations. Because of the use of lines in hockey, ice hockey teams have relatively large rosters compared to the number of players on the ice (23 for a typical NHL team, with 20 active on game day and six on the ice at any given time). Only gridiron football has a larger relative roster size (the NFL has 53 players, 46 active on gameday, 11 on the field).