Can your opponent know what cards are in your sideboard?

Ruling: Your opponent cannot see what cards are in your sideboard. They can only count the number of cards in your sideboard (either 0 or 8 cards).

Nuances:

• It is recommended to shuffle your entire sideboard into your main deck and then remove what you don't want when sideboarding, so your opponent doesn't know how many cards you swapped
• You are entitled to look at your own sideboard and count your opponent's sideboard at any time
• The sideboard must be distinguishable from other cards, but there are no specific rules about whether it must be visible on the playmat (it can be in your deck box)