Side Deck

From Yu-Gi-Oh!

Your Side Deck is a collection of exactly 15 Cards, separate from your Main Deck. The Side Deck is used only between Duels, to alter the contents of your Main Deck 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.

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).

However, a Side Deck is not compulsory, in which case, a value of 0 for a Side Deck is also considered legal. At present, you must have either 0 or 15 cards in your Side Deck, but the OCG appears to be moving towards variable numbers (i.e. between 0 and 15) being allowed.

You cannot put Fusion Monsters in your Side Deck^ref.