A Side Deck (サイドデッキ, Saidodekki) is a collection of between 1 and 15 cards, separate from the Main Deck. The Side Deck is used in Matches between Duels, to alter the contents of the Main or Extra Deck. Previously, the Side Deck could not be used to alter the contents of the Extra Deck, though this has been changed. This is done to better suit a strategy against an opponent's Deck.
For example, a Burn Deck, a 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".
Common Side Deck cards include various forms of Spell and Trap removal, such as "Dust Tornado" and cards that can directly counter common strategies, such as "Prohibition", "Chain Destruction" and Royal Prison.
Since a Side Deck is not compulsory, a value of 0 for a Side Deck is considered legal. In both the OCG and the TCG, any value between 0 and 15 inclusive is permissible.