Rule 708.5

Anything that asks for a particular characteristic of a split card while it’s in a zone other than the stack gets two answers (one for each of the split card’s two halves). Anything that asks for a particular characteristic of a fused split spell gets two answers. Anything that asks for a particular characteristic of a split card while it’s a spell on the stack or that asks for the converted mana cost of a fused split spell gets one answer. Example: Infernal Genesis has an ability that reads, "At the beginning of each player’s upkeep, that player puts the top card from his or her library into his or her graveyard. He or she then puts X 1/1 black Minion creature tokens onto the battlefield, where X is that card’s converted mana cost." If the top card of your library is Assault/Battery, which has converted mana costs of 1 and 4, when this ability resolves, you simultaneously put one and four creature tokens onto the battlefield, for a total of five. Example: Spell Blast says "Counter target spell with converted mana cost X." If a player wants to cast Spell Blast and choose the fused split spell Breaking/Entering, which has mana costs {U}{B} and {4}{B}{R}, as the target, that player must choose 8 as the value of X.

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