The Side Deck is a special deck used in tournaments and match-play and is only ever referenced between games in a match.
In Constructed events, side decks can contain a maximum of fifteen cards. Side decks may contain ruler cards and magic stone cards as well. The combined main deck and side deck may not include more than four copies of any card. The combined magic stone deck and side deck may not include more than four copies of any special magic stone cards. Non-special magic stone cards have no limit and can be included in any number of copies.
Limited side decks consist of all cards not included in the main deck.
After games in a match, players may exchange cards in their main decks and cards in their side decks.
In Constructed formats, cards must be exchanged so that the number of cards in the main deck, the magic stone deck and the side deck remain the same.
In Limited formats, cards may be exchanged freely between deck and side deck as long as the main deck contains at least 20 cards and the magic stone deck contains at least 8 cards. Main decks and magic stone decks must be restored to their original configuration for the beginning of each new match. Decks may not be altered between rounds.
Players may not look at their side decks during a game unless a card effect specifically instructs them to do so.