Maximum of 3 Rule

Between your Main Deck, Extra Deck and Side Deck, you may only have 3 copies of 1 card of the same name. Here are some examples:

Example 1

 * The card effects of "Harpie Lady", "Cyber Harpie Lady", "Harpie Lady 1", "Harpie Lady 2", and "Harpie Lady 3" all state that their name is also "Harpie Lady", therefore they all share the same card name "Harpie Lady", so you can only have a maximum of 3 between these card names. For example:
 * 3 copies of "Harpie Lady", no copies of the others.
 * 1 copy of "Harpie Lady 1", 1 copy of "Harpie Lady 2", 1 copy of "Harpie Lady 3", no copies of either "Harpie Lady" or "Cyber Harpie Lady".
 * 2 copies of "Harpie Lady 2", 1 copy of "Harpie Lady 3", no copies of either "Harpie Lady 1", "Harpie Lady", or "Cyber Harpie Lady".

"Harpie Queen", on the other hand, only has it's name treated as "Harpie Lady" while it is face-up on the field, or in the Graveyard, but not in the Main Deck, so you can have 3 copies of this card, in addition to any legal combination of the above cards.


 * The rule also applies to "A Legendary Ocean". Its card effect states its name is treated as "Umi" at all times, so you can only have 3 cards in total between these two.  You cannot have 3 copies of "A Legendary Ocean" and 3 copies of "Umi" in your deck.

Example 2

 * Proto-Cyber Dragon only has it's name treated as "Cyber Dragon" while it is face-up on the field. It is not illegal to have 4 or 5 cards on the field that all have the same card name, if the name change is only applicable while on the field and not in the Main Deck. So, you may theoretically have 3 copies of "Cyber Dragon", and 3 copies of "Proto-Cyber Dragon" in your Main Deck. However, Cyber Dragon is limited for its power, reducing the amount you may have in a deck.

Example 3

 * Cards that are similar in name. For example:
 * "Battle Ox" and "Enraged Battle Ox" have different card names.
 * "Obnoxious Celtic Guard" and "Celtic Guardian" have different card names.
 * "Swift Gaia the Fierce Knight" and "Gaia the Fierce Knight" have different card names.
 * "Black Luster Soldier" and "Black Luster Soldier - Envoy of the Beginning" have different card names.

In the above cases, a duelist may theoretically have up to 3 of each card above in their Deck. These cards are seen as Retrained from the original Normal Monster or Ritual Monster version of the card

Example 4

 * Cards that are stored in the Extra Deck and have the same name as a card that exists in either the Main Deck or Side Deck:
 * Neo-Spacian Marine Dolphin is stored in the Extra Deck, but by its effect its name is treated the same as "Neo-Spacian Aqua Dolphin" and therefore you cannot have more than three of a combination of these cards, despite them being stored in separate Decks.
 * Neo-Spacian Twinkle Moss follows the same rule.