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Your Side Deck is a collection of between 1 and 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.
However, a Side Deck is not compulsory, in which case, a value of 0 for a Side Deck is also considered legal. In both the OCG and the TCG, any value between 0 and 15 inclusive is permissible.