The National Hockey League (NHL) is a professional ice hockey league with 31 teams founded in 1917. Each team can select a captain, who has the "sole privilege of discussing with the Referee any questions relating to interpretation of rules which may arise during the progress of a game". Each team is also allowed to pick alternate captains, who work when the captain is not on the ice. Captains have to wear the letter "C" on their uniform to show who they are; alternate captains wear the letter "A". Both letters are in height.