Main Deck

A Duelist's Main Deck is the pile of at least 40 to 60 cards, placed on the Deck Zone. The Main Deck is, essentially, all the cards the duelist will be playing with in Duel. This Deck may contain all of the Trap, Spell and Monster Cards one duelist may be using, with the exception of Fusion Monsters and Synchro Monsters, which are placed on the opposite end of the main Deck Zone in the Extra Deck. Whenever a card effect is activated that lets you search your Deck and select a card(s) to place in your Hand, on the field or in the Graveyard, you should always shuffle your Deck to maintain fair play.

Most Decks contain approximately 40 cards, since 40 is the minimum amount of cards a Deck must have in order to be legal (not including cards in the Extra Deck). Most experienced Duelists will argue that you should stick to the minimum because it increases the chances of you drawing your best card(s). Also another point of view says that a large deck with cards that allow you to draw the ones you need, like: Terraforming or Dark Magic Curtain can also work well. If a deck focuses on one strategy and it holds no real critical cards for it work, like a burn deck perhaps, then a lot of cards can be helpful for more 'ammo'.

New rulings from Konami states that no cards can be placed below the line of your main deck, also you cannot turn it sideways and so on. If you are confused on how your Main Deck should be placed, then look at a playmat from structure decks to see how you should place your cards.