403.1. In certain competitions, players may have a group of additional cards (outside of their starting configuration) called a sideboard.
403.2. In constructed format’s, a player’s sideboard must be exactly 0 or 8 cards.
403.3. A sideboard can consist only of valid Main Deck cards.
403.4. Limits on copies of named cards apply to the combination of Main Deck and sideboard.
403.5. Players may use those cards to modify their deck after the first game of a match.
403.6. Players may not use sideboard cards in the first game of a match.
403.7. Players may count the cards in their opponents’ sideboards at any time.
403.8. A player may look at their own sideboard at any time, but must keep it distinguishable from other cards.
403.9. A player’s deck and sideboard must be returned to their registered states before the first game of the next match.
403.10. If a match skips the first game due to a penalty or an intentional draw, neither player qualifies for 403.5.
403.11. If a game restarts due to an in-game effect or due to corrective action by judges, the state of each player’s deck and sideboard must remain the same in the restarted game.