It is used to customize a match strategy against an opponent by enabling a player to change the composition of the playing deck.
[1][2] In Magic: The Gathering, a player may have a playing deck and an optional sideboard or "side".
[4][5] Previous versions of the rules required the optional sideboard to contain exactly 15 cards,[6] and for players to agree to their use before a match.
[7] This rule was changed with the prerelease of the Magic 2014 core set, and became standard effective 13 July 2013.
[14] The number of cards removed from the playing deck need not be the same as the number of cards added to it from the sideboard, but the changes must satisfy the conditions for minimum playing deck size and maximum sideboard size.
[2] The sideboard must be set aside before the playing deck is shuffled,[15] and those cards are considered to be outside the game.
[17][18][13] A sideboard can lack versatility because of the limited number of cards it can contain and the diversity of decks that can be constructed.
[20] Cards typically chosen for a sideboard include those that "fulfill multiple purposes or deal with more than one threat".
[30][31] The exchange enables a player to modify the playing deck to suit a game strategy against the opponent.
[41] Most competitive World of Warcraft tournaments require players to register the list of cards in the side deck with the organizers.