Maximum of 3 Rule
The cards "Harpie Lady", "Cyber Harpie Lady", "Harpie Lady 1", "Harpie Lady 2", and "Harpie Lady 3" each have a name condition stating that their name is also "Harpie Lady". Therefore you can only have a total of three of these cards in your Deck. For example:
- Three copies of "Harpie Lady", no copies of the others.
- One copy of "Harpie Lady 1", one copy of "Harpie Lady 2", one copy of "Harpie Lady 3", no copies of either "Harpie Lady" or "Cyber Harpie Lady".
- Two copies of "Harpie Lady 2", one 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 its 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 three copies of "Harpie Queen" in addition to any legal combination of the above cards.
The rule also applies to "A Legendary Ocean". Its condition states its name is treated as "Umi" at all times, so you can only have three cards in total between these two. You cannot have three copies of "A Legendary Ocean" and three copies of "Umi" in your deck.
"Proto-Cyber Dragon" only has its name treated as "Cyber Dragon" while it is face-up on the field. It is not illegal to have more than three cards on the field with the same name; the rule only applies while the cards are in the Deck. Therefore, you can have three copies of "Cyber Dragon" and three copies of "Proto-Cyber Dragon" in your Main Deck.
Some cards are similar in name and stats and depict the same monster. For example:
- "Enraged Battle Ox" and "Battle Ox"
- "Obnoxious Celtic Guard" and "Celtic Guardian"
- "Swift Gaia the Fierce Knight" and "Gaia the Fierce Knight"
- "Black Luster Soldier - Envoy of the Beginning" and "Black Luster Soldier"
In the above cases, a player may have up to three of each card above in their Deck. While these cards may be styled as "Retrained" versions of the older card, they do not share the same name.
- "Neo-Spacian Marine Dolphin" is stored in the Extra Deck, but by its condition its name is treated as "Neo-Spacian Aqua Dolphin" at all times. 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 with "Neo-Spacian Glow Moss".