Extra Deck

The Fusion Deck (Soon to be called Extra Deck in the Japanese version) is a Deck where the Fusion Monsters are stored during a Duel. They are separate from the Main Deck, and you can search through it to find a Fusion Monster that you need. There is currently no limit as to how many cards you can use in the Fusion Deck, but the OCG appears to be moving toward a 15-card limit (which is also enforced in many Video Games)

When a Fusion Monster on the field is returned to its owner's hand or Deck, it is instead sent to the Fusion Deck, since Fusion Monsters cannot exist in either the Hand or the Main Deck.

There is no limit as to how big your Fusion Deck can be, although the "Maximum of 3 Rule", for cards of the same name still applies.

The contents of your Fusion Deck are not public knowledge to your opponent, and only you may look through its contents.

With the release of the Synchro Monsters, which also go into the Fusion Deck, the deck name has now officially been changed to "Extra Deck".