A Duelist's Main Deck is the pile of cards that is placed on the Deck Zone. The Main Deck is, essentially, all the cards the Duelist will be playing with in a Duel. This Deck contains all of the cards (Trap, Spell and Monster) that the Duelist may use, excluding Fusion, Synchro, and Xyz monsters, which are kept in a seperate "Extra Deck".
Most card effects that allow you search your Deck and select one or more card(s) normally include shuffling your Deck, this is to maintain fair play.
To be legal, the Main Deck must be between 40 to 60 cards with most decks containing approximately 40 cards. Most experienced Duelists will argue that you should stick to the minimum because it increases the chances of you drawing your best card(s). Another point of view is that a large Deck, with cards that allow you to search out ones you need ("Terraforming" or "Dark Magic Curtain") may also work well. The more cards you include in a deck, the more variety you can get from a single deck, allowing you to possibly change strategy without much difficulty and maintain that change. The downside of this is the deck may not be consistent, and a bad hand is inevitable.
Larger decks usually contain multiple copies of the same card, as to increase the chances of drawing that card, while smaller decks contain only one to two copies, as they are not critical to maintaining a strategy. Larger decks tend to not rely on Extra Deck monsters, instead having most of their strategy in what they search for / draw from the deck itself. Certain cards like "Mystic Tomato" or "Gold Sarcophagus" are also more useful in a larger Deck, searching out and bringing to the field / hand for easy use.
Smaller decks are more easily stopped by Milling strategies, as the cards lost aren't replaceable and a duelist may be more likely to Deck Out, losing by default. Smaller decks tend to focus on one strategy and to defeat the opponent more quickly, and rely on Extra Deck monsters as the bulk of their forces, supporting them with a few Trap Cards and using Spell Cards to further their strategy and control.
However, larger decks are less predictable, which can be beneficial or disadvantageous to both the player and the opponent. Also, large decks are highly unfavorable in competitive play due to their unpredictability.
Rulings from Konami regarding the Main Deck include:
- No cards can be placed below the line of your Main Deck.
- You cannot turn it sideways.