Triggered ability

A triggered ability is an ability that automatically does something when a certain event occurs or a set of conditions is met (the latter is called a state-triggered ability).

Description
A triggered ability can be identified by the words "when," "whenever," or "at", which will usually be found at the start of the ability. The phrase that contains one of those words lists the conditions (the trigger event or trigger state) where the ability will trigger. When it does, the ability goes on the stack (unless it's a mana ability), and responses can be played.

Rules
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Trigger
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Common cases
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Chapter ability
A chapter ability, introduced in Dominaria, is a triggered ability that triggers when a lore counter that is put on a Saga causes the number of lore counters on the Saga to become equal to or greater than the ability's chapter number. Chapter abilities are put onto the stack and may be responded to.

Examples
A triggered ability can be a mana ability, which means that it doesn't go on the stack, but resolves immediately. A triggered ability is a mana ability if it has the capability to produce mana, doesn't target anything, and triggers by activating another (activated) mana ability.

Effects can also create a delayed trigger ability, which waits until the next time its conditions are fulfilled, then triggers usually once. (Delayed triggers that state a duration will trigger any time, within that duration, that their trigger event occurs.)

Using a triggered ability is not optional. When triggered, the controller can't refuse to put the ability on the stack. Nor can he or she choose an illegal target, if a target is required. If a targeted ability has no legal targets in play, it still triggers and goes on the stack, but is taken off immediately.

Triggered abilities are cumulative, and multiple instances of the same triggered ability on a permanent will each trigger separately.

Chapter abilities were introduced in Dominaria.