Typically, an NHL team dresses twelve forwards along four lines and three pairs of defencemen, though some teams elect to dress a seventh defenceman, or a thirteenth forward.
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.
Only gridiron football has a larger relative roster size (the NFL has 53 players, 46 active on gameday, 11 on the field).
The top pairing consists of the two strongest defenders on the team, and may play up to 30 minutes per game.
[2] Coaches may also choose to pair a more offensively-minded defender with a more defensive player.