D.11.1- A sideboard is a set of cards which are part of the tournament along with your deck.
D.11.2- A sideboard in all constructed deck formats is exactly 15 cards. [Tournament Rules 1997/10/01]
D.11.3- Sideboards are optional. If a player chooses not to use one, they must inform their opponent prior to the start of the match. [Tournament Rules 1997/10/01]
D.11.4- In constructed deck tournaments, after each duel, a player may swap cards in their deck with cards in their sideboard. This is a 1-for-1 swap so both the deck and sideboard remain the same size. [Tournament Rules 1997/10/01]
D.11.5- In sealed deck tournaments, all the remaining cards not in your play deck are considered your sideboard. Cards may be moved between your deck and your sideboard as your choose as long as your deck size doesn't fall below the required limit. [Tournament Rules 1997/10/01]
D.11.6- Players cannot look through their sideboards during play. [Tournament Rules 1998/09/01]