A Side Deck (Japanese: サイドデッキ Saidodekki) is a collection of between 1 and 15 cards, separate from the Main Deck. In between Duels of a Match, players may exchange cards from their Side Deck with cards in their Main or Extra Deck to better suit a strategy against an opponent's Deck.
The Side Deck is invaluable in tournament play. Facing certain Decks can be a walk in the park or a nightmare depending on the contents of your Side Deck. For example, a Burn Deck based around dealing direct damage through Card effects and Spell Cards could be easily countered by including several cards to prevent Life Point damage, such as "Des Wombat" or "Nature's Reflection". In addition, it can be a method of shoring up a Deck's weaknesses against other Decks that can easily exploit them, by adding cards to the DEck that provide extra defense against them or removing cards from the Deck that are weak against them. Familiarizing oneself with the most popular Decks is a great way to know what cards to put in a Side Deck.
Common Side Deck cards include various forms of Spell and Trap removal, such as "Twin Twisters", and cards that can directly counter common strategies, such as "Prohibition", "Imperial Iron Wall" and "Royal Prison".
Since a Side Deck is not compulsory, a value of 0 for a Side Deck is considered legal. Any value between 0 and 15 inclusive is permissible.
- Yugioh-card.com Change in Rulebook.