How do split cards work?

I saw these cards on the Internet and quickly became curious: How in the world do they work? Do you pick one of them to play and then discard the card, or can you play both? How does their color and mana-cost duality function with cards that depend on them? When were these cards added to MTG?

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There are similar cards called fuse cards that allow you to cast both, but not these. – KRyan Mar 19 '15 at 3:47

They work as if they only had one of the sides attributes while on the Stack, and as if they had both sides' qualities in every other zone.

If an effect is asking for a particular characteristic, it gets two answers. For example, Infernal Genesis asks you to reveal the top card of your library and add a number of 1/1 tokens to the battlefield based on the converted mana cost of the revealed card, if you revealed Hit//Run you would get 8 tokens, because the converted mana cost of the card would have two answers 3 and 5.

If an effect is performing a comparison, it only gets one answer. If either side would return a Yes answer when performing the comparison, then you would get a single Yes. For example, if you used Sunforger to search your library for a card that was red or white and had a converted mana cost of 4 or less, you could search out Wax/Wane (because for one of the sides, you would get a yes - Wane is white, and has a CMC of 4 or less) and still cast the Green side (Wax) of the card (only the search is restricted by color and CMC).

When casting a split card, you choose a side and proceed to do the normal process for casting a spell. Announce the spell, choose targets, pay costs, etc.

The Split Card type was added to MtG first in Invasion Block.

708.2. In every zone except the stack, split cards have two sets of characteristics and two converted mana costs. As long as a split card is a spell on the stack, only the characteristics of the half being cast exist. The other half’s characteristics are treated as though they didn’t exist.

708.2a If a player casts a split card, that player chooses which half of that split card he or she is casting before putting it onto the stack. Only the half that is being cast is considered to be put onto the stack.

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Considering the question about whether or not you can cast both halves, it might be worthwhile to mention fuse cards in your answer. – KRyan Mar 19 '15 at 3:47

These are called "split cards". They were introduced in Invasion block and then returned during Ravnica and Time Spiral blocks.

When you cast one, you pick one side of the card to cast and then treat the other like it doesn't exist. The whole card will go in your graveyard. There's no way to cast both sides at once (unlike Entwine spells).

As far as what happens in other zones, the basic idea is that the card has two values. For instance, Hit / Run has a combined mana cost of "3 and 5" and its color are red, green, and black.

As far as what "3 and 5" really means in all sorts of situations, honestly it's probably easiest to quote the whole comp rules:

708. Split Cards

708.1. Split cards have two card faces on a single card. The back of a split card is the normal Magic card back.

708.2. In every zone except the stack, split cards have two sets of characteristics and two converted mana costs. As long as a split card is a spell on the stack, only the characteristics of the half being cast exist. The other half’s characteristics are treated as though they didn’t exist.

708.2a If a player casts a split card, that player chooses which half of that split card he or she is casting before putting it onto the stack. Only the half that is being cast is considered to be put onto the stack.

708.3. Each split card that consists of two halves with different colored mana symbols in their mana costs is a multicolored card while it’s not a spell on the stack. While it’s a spell on the stack, it’s only the color or colors of the half being cast.

708.4. Although split cards have two castable halves, each split card is only one card. For example, a player who has drawn or discarded a split card has drawn or discarded one card, not two.

708.5. An effect 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).

• 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 when this ability resolves, the game sees its converted mana cost as “1, and 4.” You get five creature tokens.

708.6. Some effects perform comparisons involving characteristics of one or more split cards in a zone other than the stack.

708.6a An effect that performs a positive comparison (such as asking if a card is red) or a relative comparison (such as asking if a card’s converted mana cost is less than 2) involving characteristics of one or more split cards in any zone other than the stack gets only one answer. This answer is “yes” if either side of each split card in the comparison would return a “yes” answer if compared individually.

708.6b An effect that performs a negative comparison (such as asking if cards have different names) involving characteristics of one or more split cards in any zone other than the stack also gets only one answer. This answer is “yes” if performing the analogous positive comparison would return a “no” answer.

708.6c If an effect performs a comparison involving multiple characteristics of one or more split cards in any zone other than the stack, each characteristic is compared separately. If each of the individual comparisons would return a “yes” answer, the whole comparison returns a “yes” answer.

• Example: Void reads, “Choose a number. Destroy all artifacts and creatures with converted mana cost equal to that number. Then target player reveals his or her hand and discards all nonland cards with converted mana cost equal to the number.” If a player casts Void and chooses 1, his or her opponent would discard Assault/Battery because the game sees its converted mana cost as “1, and 4.” The same is true if the player chooses 4. If the player chooses 5, however, Assault/Battery would be unaffected.

708.7. If an effect instructs a player to name a card and the player wants to name a split card, the player must name both halves of the split card. An object has the chosen name if it has at least one of the two names chosen this way.

Thus, we get these interactions:

If your Dark Confidant digs up Hit / Run, he'll see "3 and 5" (thanks to rule 708.5) and you lose 8 life. Brutal!

Bloodbraid Elf's cascade ability, on the other hand, is a positive characteristic comparison, so that "3 and 5" allows Hit / Run to satisfy the condition if either value would individually (see rule 708.6a). That means that, when Bloodbraid Elf finds Hit / Run, the entire card Hit / Run satisfies the condition because Hit would. When you actually go to cast Hit / Run without paying its mana cost, you go still through the normal spell casting procedure, including choosing which side you're casting. That means you can go ahead and cast Run, if you want!

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708.6a also means you can imprint Assault // Battery onto an Isochron Scepter to create 3/3 Elephants for {2} or imprint Bound // Determined to cast the Bound part at a steep 60% discount, and since the card is copied by the Scepter, repeatedly. It's almost like cheating. – ghoppe Apr 9 '12 at 1:37
@ghoppe Or Research // Development for the tournament-grade combo. :) – Alex P Apr 9 '12 at 2:20
Considering the question about whether or not you can cast both halves, it might be worthwhile to mention fuse cards in your answer. – KRyan Mar 19 '15 at 3:47