This deck consists of Synchro Monsters, Fusion Monsters and Xyz (Monster). There can only be a maximum of 15 cards in a player's Extra Deck, also there can only be 3 of each Synchro or Fusion Monster in a Extra Deck. The Extra Deck is not public knowledge, so a opponent doesn't have a right to see those cards at any point in a duel. A Synchro or Fusion card cannot exist in a player's hand or deck so when sent there they are instead placed in the Extra Deck.
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