Can you have more than 40 cards in your Main Deck in tournament play, and how does this work with the sideboarding rule requiring exactly 40 cards after siding?

Ruling: In tournament play, your Main Deck must have exactly 40 cards. In casual play, you only need at least 40 cards in your Main Deck.

Nuances:

• The tournament requirement for exactly 40 cards applies both before and after sideboarding
• The Core Rules allow more than 40 cards, but Tournament Rules override this with the exact 40 card requirement