Split cards are Magic cards with two card faces. A split card is literally "split" into two separate cards each with its own card name, art, mana cost, text, etc. In any zone except the stack, a split card has two sets of characteristics; while it's on the stack, it only has the characteristics of the half being played. This provides many interesting interactions with cards that create effects based on converted mana cost. For example, if Dark Confidant reveals a split card, you would lose life equal to the total converted mana cost of both sides. However, Venarian Glimmer needs only be set to the smaller converted mana cost, not the combined converted mana cost.
They were introduced in the Invasion block, where each side was from a different color.  In Dissension each half was a multicolored card from a different guild. In Planar Chaos, split cards were introduced where both sides were the same color (all split cards in PC are red).
From the Comprehensive 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 into play, 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 plays 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.
Notable split cards