Sentence:  Cards can only be exchanged between the playing deck and the <WSD>sideboard</WSD> on a one-to-one basis between duels or matches, and any number of cards, up to fifteen, can be exchanged at once. 
Meaning of the word marked with <WSD> tags:  A set of cards that are separate from a player's primary deck, used to customize a match strategy against an opponent by enabling a player to change the composition of the playing deck.