Sideboard

Each Magic player is allowed to have a sideboard, which is a group of additional cards Outside the game that the player may use to modify their deck between games of a match. A sideboard helps a player address the weaknesses of their deck against their opponent. For example, if a player consistently loses games against anyone who is playing red "burn" spells like Ghitu Fire and Urza's Rage, then the player may want to put four Chill (so as to "hose" red by increasing the spells' playing cost) or maybe four Ivory Mask (to prevent a player from being targeted) in their sideboard.

Description
In conventional games, each player is allowed to bring at most 7 cards for best-of-one and 15 cards for best-of-three matches (in addition to a player's main deck) to a game of Magic: The Gathering. These cards are referred to as the sideboard. In limited games, all unused cards are treated as a sideboard.

A sideboard counts as part of the player's deck, therefore any limits to the number of copies of a card that may be included in a deck take the copies in the sideboard into account. This also means that restricted cards are limited to one including the sideboard. (For example, the card Black Lotus is restricted in DCI-sanctioned Vintage Magic tournaments. This means that only one Black Lotus is allowed per deck, including sideboard, in the Vintage format.) Players are not required to have a sideboard. If a library consists of more than the minimum amount of required cards, players are still allowed to sideboard, even if their sideboard consists of no cards.

The first game of a best-of match is usually played without cards from the sideboard and is therefore called pre-sideboard game. After the first match the players are allowed to swap/add/remove cards in their deck for cards in their sideboard (aka "sideboarding"), as long as the sideboard and the main deck remain legal after the sideboarding.

Starting with the Shadows over Innistrad Pro Tour, the first two games of a best-of-five Sunday playoff match are considered pre-sideboard games.

Wishes and Companions are methods to get access to your sideboard during a game. Learn is also a mechanic that has the choice to access the sideboard, by fetching instants and sorceries that have the lesson subtype.

The Commander format normally has no sideboard. However, a companion is the only card that can exist there, if both the cards in the library and the commander card meet the companion criteria.

Dungeon s are card types introduced in Dungeons & Dragons: Adventures in the Forgotten Realms. They start exclusively in the sideboard, but do not count as cards towards your sideboard limit. When an ability instructs you to venture into the dungeon, if you don't have a dungeon card in your command zone, you take one from the sideboard, put it there and trigger the ability of the first room. You must first complete a dungeon before being able to take another dungeon card from the sideboard.

Previous rulings
Prior to the amendment of regulations in Magic 2014, the sideboard restrictions in constructed formats were more strict than the current one, as below:
 * 1) If a player used a sideboard, the sideboard had to consist of exactly 15 cards. No more, no less.
 * 2) Sideboarding had to be a 1-for-1 swap, i.e. the sideboard had to contain 15 cards, and the number of cards in main deck must be the same.

If one of the two cases above was violated in tournaments, it resulted in a game loss.

Prior to Strixhaven: School of Mages set, the sideboard for constructed formats (except for Commander) was limited to 15 cards for both best-of-one and best-of-three matches. With the release of this set, the best-of-one matches use a 7-card sideboard, companion included.

Rules
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Black

 * Deathmark - Destroy a green or white creature

Blue

 * Annul - Counter answer to decks that rely on artifacts and/or enchantments
 * Chill</c> - Hoses red decks
 * Flashfreeze</c> - Cheap red/green counter

Green

 * Guttural Response</c> - Counter target blue instant spell.
 * Naturalize</c> - Green's Disenchant</c>
 * Uktabi Orangutan</c> and Viridian Shaman</c> - 2/2 creatures that destroys an artifact when it comes in play

Red

 * Blood Moon</c> and Magus of the Moon</c> - Hoses decks that rely on many non-basic lands by turning them into mountains
 * Boil</c> and Boiling Seas</c> - Forces the opponent to waste countermagic, or lose it altogether.
 * Guttural Response</c> - Counter target blue instant spell.
 * Red Elemental Blast</c>, Pyroblast</c> and Active Volcano</c> - Strong answers to blue countermagic

White

 * Disenchant</c> - Destroy an artifact or enchantment
 * <c>Honorable Passage</c> - Prevent damage from a source, if its red then reverse that damage

Land

 * <c>Wasteland</c> - Destroy a non-basic land