The lines in hockey refer to the different lineups that teams utilize throughout the game. Each line typically consists of three forwards, a center and two wingers on either side of the center, and two defensemen. Usually, teams field four lines of forwards and 3 lines of defensemen.
Skill plays a major part in which line a player is on, but teams must also take into account the chemistry of their lines. Different specialties and body types between players on a line make a team more even and ready for any type of opposition.
Teams must also prepare powerplay lines with scorers and sometimes even a forward on the defensive line to increase their scoring chances. On the penalty kill, teams play four or three players that can clear the puck and play great defense to ensure that the other team does not take advantage of their extra player or players on the ice.
A team can have six skaters on the ice if they pull their goalie. When the goalie goes to the bench, teams will put a scorer on the ice to hopefully steal a goal without letting a puck into the empty net. This tactic is used in dire situations when a team needs to score, or when there is a delayed penalty call because the play is called dead when the other team touches the puck, so there is not much risk.
Line changes either happen during stoppages, which is much easier to coordinate, or in the middle of a play. During these fast-paced changes, coaches will have a sound or gesture to their team to signal the change. This reason is why the awareness and cohesiveness of a team is so crucial to success.
While this practice is not common in professional hockey, teams will still try to form different lines sometimes. This change can happen for a few reasons. First, teams in a slump may try different combinations to spark life into their players and create problems for other teams because there is no film on these lines. Also, coaches may plug young players into different lines to teach them the different aspects of NHL hockey and to see where they mesh on the team.
The 5th line is an expression referring to the fans of the home team. Fans can affect the game by cheering on and motivating their team or sabotage the opposing team by getting into their heads. This concept is why home ice is so coveted in the Stanley Cup Playoffs. Being able to play potentially four home games at home rather than only three is an advantage that can turn series.