A sideboard is a group of additional cards the player may use to modify his or her deck between games of a match. The player may use these cards in his or her main deck during all games after the first one in a match.
It is advisable for a player to have their sideboard sleeved in the same manner as their main deck in order to quickly swap cards.
Before each game begins, players must present their sideboard (if any) face down. Opponents may count the number of cards in their opponent’s sideboard at any time. Players are not required to reveal how many cards they have swapped from their main deck to their sideboard.
In the U.S. most players do not present their sideboard, instead leave it in their deck box, separate from their mainboard. This, like requiring opponents to shuffle as opposed to cutting a deck, is required but not strictly enforced. Deck/Decklist Problem in the IPG also tells us that any other cards kept with the sideboard are considered part of the sideboard. This is one of the common occurrences of Deck/Decklist Problems and why it is important for players to keep their deck separate from any other cards, for example last night’s draft rares. To avoid a penalty players can present the sideboard, which is common in some countries.
Other items (token cards, double-faced card represented in the deck by a checklist card, etc.) should be kept separate from the sideboard during game play.
During a game, players may look at their own sideboard, keeping it clearly distinguishable from other cards at all times. If a player gains control of another player, he or she may not look at that player’s sideboard, nor may he or she have that player access his or her sideboard.
It is very important that players keep the cards separate from their main deck. If at some point the sideboard becomes intermingled with the main deck there is no way to verify the legality of the deck and penalties may apply. Please note the new change! We no longer allow a player to look at another player’s sideboard while they control them. Players may still look at their own sideboard at any time however.
The deck and sideboard must each be returned to their original compositions before the first game of each match.
This rule applies to all Constructed format events, including ones run at Regular Rules Enforcement Level, not only those that use decklists. This is only required for Limited format events if deck registration sheets are being used, otherwise the events are considered to be continuous construction.
Restrictions on the composition and use of a sideboard can be found in the deck construction rules for a particular format type.
Most Constructed formats use a maximum 15 card sideboard. In Limited format events the cards that are in your card pool but not in the main deck are considered the sideboard, along with an unlimited number of basic lands.
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, neither player may use cards from his or her sideboard for the next game in the match.
In this case there has not been a “first game” for this match so there is no sideboarding as the next game will be the “first game”. Note that this exception applies when a game loss penalty is given before the first game has started, not the second or any subsequent games. A Deck/Decklist Problem or Marked Cards upgraded Game Loss penalty discovered during a deck check are the common ways this can happen.
If players restart a game due to an in-game effect, the composition of their decks must remain the same for the restarted game.
Currently Karn Liberated’s ultimate is the only ability that can restart a game. See CR 714 for additional rules on restarting the game.
Certain cards refer to “a (card or cards) you own from outside the game.” In tournament play, a card “you own from outside the game” is a card in that player’s sideboard.
In order to prevent any debates or weird rulings over what defines “a card you own”, tournament play now strictly goes by this rule. This usually comes up in the form of “Wish” cards, which let you put such a card into your hand and are frequently used in Modern and Legacy decks.