A sideboard is a group of additional cards the player may use to modify their deck between games of a match. The player may use these cards in their main deck during all games after the first one in a match.
Players must begin each match with the deck and sideboard in their original compositions.
Restrictions on the composition and use of a sideboard can be found in the deck construction rules for a particular format type.
If a penalty causes a player to lose the first game in a match before that game has begun, or the first game is intentionally drawn before any cards are played, players may use cards from their sideboard for the next game in the match.
Certain cards refer to “a (card or cards) from outside the game.” In tournament play, these are cards in that player’s sideboard.