What should a sideboard contain?
Perhaps cards that cover your weaknesses against certain decks, or cards that punish certain popular decks. Some sideboards have a different strategy than the maindeck to catch your opponent off-guard. But remember – your sideboard and your deck combined can’t contain more than 4 copies of a single card (excluding basic lands). That means you can’t have 3 Shocks in the maindeck and 4 in the sideboard – that’s a total of 7 Shocks, and thus illegal.
