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Split second is a static ability that functions only while the spell with split second is on the stack. “Split second” means “As long as this spell is on the stack, players can’t cast other spells or activate abilities that aren’t mana abilities.”

Players may activate mana abilities and take special actions while a spell with split second is on the stack. Triggered abilities trigger and are put on the stack as normal while a spell with split second is on the stack.

Multiple instances of split second on the same spell are redundant.

Fuse is a static ability found on some split cards (see rule 709, “Split Cards”) that applies while the card with fuse is in a player’s hand. If a player casts a split card with fuse from their hand, the player may choose to cast both halves of that split card rather than choose one half. This choice is made before putting the split card with fuse onto the stack. The resulting spell is a fused split spell.

A fused split spell has the combined characteristics of its two halves. (See rule 709.4.)

The total cost of a fused split spell includes the mana cost of each half.

As a fused split spell resolves, the controller of the spell follows the instructions of the left half and then follows the instructions of the right half.