A triggered ability is an ability that automatically does something when a certain set of conditions are met.
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) where the ability will trigger. When it does, most triggers go on the stack, and responses can be played.
 Comprehensive Rules
From the Comprehensive Rules (Innistrad)
- 603. Handling Triggered Abilities
- 603.1. Triggered abilities have a trigger condition and an effect. They are written as "[Trigger condition], [effect]," and begin with the word "when," "whenever," or "at." They can also be expressed as "[When/Whenever/At] [trigger event], [effect]."
- 603.2. Whenever a game event or game state matches a triggered ability's trigger event, that ability automatically triggers. The ability doesn't do anything at this point.
- 603.2a Because they aren't cast or activated, triggered abilities can trigger even when it isn't legal to cast spells and activate abilities. Effects that prevent abilities from being activated don't affect them.
- 603.2b When a phase or step begins, all abilities that trigger "at the beginning of" that phase or step trigger.
- 603.2c An ability triggers only once each time its trigger event occurs. However, it can trigger repeatedly if one event contains multiple occurrences. See also rule 509.4.
Example: A permanent has an ability whose trigger condition reads, "Whenever a land is put into a graveyard from the battlefield, ...." If someone casts a spell that destroys all lands, the ability will trigger once for each land put into the graveyard during the spell's resolution.
- 603.2d Some trigger events use the word "becomes" (for example, "becomes tapped" or "becomes blocked"). These trigger only at the time the named event happens—they don't trigger if that state already exists or retrigger if it persists. Similarly, they don't trigger if an object enters a zone in that state.
Example: An ability that triggers when a permanent "becomes tapped" triggers only when the status of a permanent that's already on the battlefield changes from untapped to tapped.
- 603.2e If a triggered ability's trigger condition is met, but the object with that triggered ability is at no time visible to all players, the ability does not trigger.
- 603.2f An ability triggers only if its trigger event actually occurs. An event that’s prevented or replaced won’t trigger anything.
Example: An ability that triggers on damage being dealt won't trigger if all the damage is prevented.
- 603.3. Once an ability has triggered, its controller puts it on the stack as an object that's not a card the next time a player would receive priority. See rule 116, "Timing and Priority." The ability becomes the topmost object on the stack. It has the text of the ability that created it, and no other characteristics. It remains on the stack until it's countered, it resolves, a rule causes it to be removed from the stack, or an effect moves it elsewhere.
- 603.3a A triggered ability is controlled by the player who controlled its source at the time it triggered, unless it's a delayed triggered ability. To determine the controller of a delayed triggered ability, see rules 603.7d–f.
- 603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities he or she controls on the stack in any order he or she chooses. (See rule 101.4.) Then the game once again checks for and resolves state-based actions until none are performed, then abilities that triggered during this process go on the stack. This process repeats until no new state-based actions are performed and no abilities trigger. Then the appropriate player gets priority.
- 603.3c If a triggered ability is modal, its controller announces the mode choice when he or she puts the ability on the stack. If one of the modes would be illegal (due to an inability to choose legal targets, for example), that mode can't be chosen. If no mode can be chosen, the ability is removed from the stack. (See rule 700.2.)
- 603.3d The remainder of the process for putting a triggered ability on the stack is identical to the process for casting a spell listed in rules 601.2c–d. If a choice is required when the triggered ability goes on the stack but no legal choices can be made for it, or if a rule or a continuous effect otherwise makes the ability illegal, the ability is simply removed from the stack.
- 603.4. A triggered ability may read "When/Whenever/At [trigger event], if [condition], [effect]." When the trigger event occurs, the ability checks whether the stated condition is true. The ability triggers only if it is; otherwise it does nothing. If the ability triggers, it checks the stated condition again as it resolves. If the condition isn't true at that time, the ability is removed from the stack and does nothing. Note that this mirrors the check for legal targets. This rule is referred to as the "intervening 'if' clause" rule. (The word "if" has only its normal English meaning anywhere else in the text of a card; this rule only applies to an "if" that immediately follows a trigger condition.)
Example: Felidar Sovereign reads, "At the beginning of your upkeep, if you have 40 or more life, you win the game." Its controller's life total is checked as that player's upkeep begins. If that player has 39 or less life, the ability doesn't trigger at all. If that player has 40 or more life, the ability triggers and goes on the stack. As the ability resolves, that player's life total is checked again. If that player has 39 or less life at this time, the ability is removed from the stack and has no effect. If that player has 40 or more life at this time, the ability resolves and that player wins the game.
- 603.5. Some triggered abilities' effects are optional (they contain "may," as in "At the beginning of your upkeep, you may draw a card"). These abilities go on the stack when they trigger, regardless of whether their controller intends to exercise the ability's option or not. The choice is made when the ability resolves. Likewise, triggered abilities that have an effect "unless" something is true or a player chooses to do something will go on the stack normally; the "unless" part of the ability is dealt with when the ability resolves.
- 603.6. Trigger events that involve objects changing zones are called "zone-change triggers." Many abilities with zone-change triggers attempt to do something to that object after it changes zones. During resolution, these abilities look for the object in the zone that it moved to. If the object is unable to be found in the zone it went to, the part of the ability attempting to do something to the object will fail to do anything. The ability could be unable to find the object because the object never entered the specified zone, because it left the zone before the ability resolved, or because it is in a zone that is hidden from a player, such as a library or an opponent's hand. (This rule applies even if the object leaves the zone and returns again before the ability resolves.) The most common zone-change triggers are enters-the-battlefield triggers and leaves-the-battlefield triggers.
- 603.6a Enters-the-battlefield abilities trigger when a permanent enters the battlefield. These are written, "When [this object] enters the battlefield, ..." or "Whenever a [type] enters the battlefield, ..." Each time an event puts one or more permanents onto the battlefield, all permanents on the battlefield (including the newcomers) are checked for any enters-the-battlefield triggers that match the event.
- 603.6b Continuous effects that modify characteristics of a permanent do so the moment the permanent is on the battlefield (and not before then). The permanent is never on the battlefield with its unmodified characteristics. Continuous effects don't apply before the permanent is on the battlefield, however (see rule 603.6e).
Example: If an effect reads "All lands are creatures" and a land card is played, the effect makes the land card into a creature the moment it enters the battlefield, so it would trigger abilities that trigger when a creature enters the battlefield. Conversely, if an effect reads "All creatures lose all abilities" and a creature card with an enters-the-battlefield triggered ability enters the battlefield, that effect will cause it to lose its abilities the moment it enters the battlefield, so the enters-the-battlefield ability won't trigger.
- 603.6c Leaves-the-battlefield abilities trigger when a permanent moves from the battlefield to another zone, or when a phased-in permanent leaves the game because its owner leaves the game. These are written as, but aren't limited to, "When [this object] leaves the battlefield, ..." or "Whenever [something] is put into a graveyard from the battlefield, ..." An ability that attempts to do something to the card that left the battlefield checks for it only in the first zone that it went to. An ability that triggers when a card is put into a certain zone "from anywhere" is never treated as a leaves-the-battlefield ability, even if an object is put into that zone from the battlefield.
- 603.6d Normally, objects that exist immediately after an event are checked to see if the event matched any trigger conditions. Continuous effects that exist at that time are used to determine what the trigger conditions are and what the objects involved in the event look like. However, some triggered abilities must be treated specially because the object with the ability may no longer be on the battlefield, may have moved to a hand or library, or may no longer be controlled by the appropriate player. The game has to "look back in time" to determine if these abilities trigger. Leaves-the-battlefield abilities, abilities that trigger when a permanent phases out, abilities that trigger when an object that all players can see is put into a hand or library, abilities that trigger specifically when an object becomes unattached, abilities that trigger when a player loses control of an object, and abilities that trigger when a player planeswalks away from a plane will trigger based on their existence, and the appearance of objects, prior to the event rather than afterward.
Example: Two creatures are on the battlefield along with an artifact that has the ability "Whenever a creature dies, you gain 1 life." Someone plays a spell that destroys all artifacts, creatures, and enchantments. The artifact's ability triggers twice, even though the artifact goes to its owner's graveyard at the same time as the creatures.
- 603.6e Some permanents have text that reads "[This permanent] enters the battlefield with ...," "As [this permanent] enters the battlefield ...," "[This permanent] enters the battlefield as ...," or "[This permanent] enters the battlefield tapped." Such text is a static ability—not a triggered ability—whose effect occurs as part of the event that puts the permanent onto the battlefield.
- 603.6f Some Auras have triggered abilities that trigger on the enchanted permanent leaving the battlefield. These triggered abilities can find the new object that permanent card became in the zone it moved to; they can also find the new object the Aura card became in its owner's graveyard after state-based actions have been checked. See rule 400.7.
- 603.7. An effect may create a delayed triggered ability that can do something at a later time. A delayed triggered ability will contain "when," "whenever," or "at," although that word won't usually begin the ability.
- 603.7a Delayed triggered abilities come from spells or other abilities that create them on resolution, or are created as the result of a replacement effect being applied. That means a delayed triggered ability won't trigger until it has actually been created, even if its trigger event occurred just beforehand. Other events that happen earlier may make the trigger event impossible.
Example: Part of an effect reads "When this creature leaves the battlefield," but the creature in question leaves the battlefield before the spell or ability creating the effect resolves. In this case, the delayed ability never triggers.
Example: If an effect reads "When this creature becomes untapped" and the named creature becomes untapped before the effect resolves, the ability waits for the next time that creature untaps.
- 603.7b A delayed triggered ability will trigger only once—the next time its trigger event occurs—unless it has a stated duration, such as "this turn."
- 603.7c A delayed triggered ability that refers to a particular object still affects it even if the object changes characteristics. However, if that object is no longer in the zone it's expected to be in at the time the delayed triggered ability resolves, the ability won't affect it. (Note that if that object left that zone and then returned, it's a new object and thus won't be affected. See rule 400.7.)
Example: An ability that reads "Exile this creature at the beginning of the next end step" will exile the permanent even if it's no longer a creature during the next end step. However, it won't do anything if the permanent left the battlefield before then.
- 603.7d If a spell creates a delayed triggered ability, the source of that delayed triggered ability is that spell. The controller of that delayed triggered ability is the player who controlled that spell as it resolved.
- 603.7e If an activated or triggered ability creates a delayed triggered ability, the source of that delayed triggered ability is the same as the source of that other ability. The controller of that delayed triggered ability is the player who controlled that other ability as it resolved.
- 603.7f If a static ability generates a replacement effect which causes a delayed triggered ability to be created, the source of that delayed triggered ability is the object with that static ability. The controller of that delayed triggered ability is the same as the controller of that object at the time the replacement effect was applied.
- 603.8.Some triggered abilities trigger when a game state (such as a player controlling no permanents of a particular card type) is true, rather than triggering when an event occurs. These abilities trigger as soon as the game state matches the condition. They'll go onto the stack at the next available opportunity. These are called state triggers. (Note that state triggers aren't the same as state-based actions.) A state-triggered ability doesn't trigger again until the ability has resolved, has been countered, or has otherwise left the stack. Then, if the object with the ability is still in the same zone and the game state still matches its trigger condition, the ability will trigger again.
Example: A permanent's ability reads, "Whenever you have no cards in hand, draw a card." If its controller plays the last card from his or her hand, the ability will trigger once and won't trigger again until it has resolved. If its controller casts a spell that reads "Discard your hand, then draw that many cards," the ability will trigger during the spell's resolution because the player's hand was momentarily empty.
- 603.9. Some triggered abilities trigger specifically when a player loses the game. These abilities trigger when a player loses or leaves the game, regardless of the reason, unless that player leaves the game as the result of a draw. See rule 104.3.
- 603.10. Some objects have a static ability that's linked to a triggered ability. (See rule 607, "Linked Abilities.") These objects combine both abilities into one paragraph, with the static ability first, followed by the triggered ability. A very few objects have triggered abilities which are written with the trigger condition in the middle of the ability, rather than at the beginning.
Example: An ability that reads "Reveal the first card you draw each turn. Whenever you reveal a basic land card this way, draw a card" is a static ability linked to a triggered ability.
- Drekavac says "When Drekavac comes into play, sacrifice it unless you discard a noncreature card." The trigger event is when Drekavac enters the in-play zone, by whatever means. As soon as it does, this ability triggers and goes on the stack. When it resolves, you must discard or sacrifice the Drekavac.
A triggered ability can be a mana ability, which means that it doesn't go on the stack, but resolves immediately. Triggered mana abilities have to have the capability to produce mana, and have to trigger off of mana abilities.
- Wild Growth says "Whenever enchanted land is tapped for mana, its controller adds to his or her mana pool." Any time the enchanted land's mana ability is activated, Wild Growth will add its own mana at the same time, without the player needing to pass priority.
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.)
- Ignorant Bliss says "Remove all cards in your hand from the game face down. At end of turn, return those cards to your hand, then draw a card." The delayed triggered effect waits until the next end of turn step, then goes on the stack. If it resolves, you return the removed cards to your hand and draw a new card. If it doesn't resolve, (such as being countered with Voidslime), those cards will remain removed from the game indefinitely, and you don't get to draw a card.
Using triggered abilities are 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.
- Nekrataal says "When Nekrataal comes into play, destroy target nonartifact, nonblack creature." If the only nonartifact, nonblack creatures in play are ones you control, you must choose one of them to be targeted by Nekrataal's ability. Only if there are only black and/or artifact creatures in play will Nekrataal's ability be taken off the stack.
Triggered abilities are cumulative, and multiple instances of the same triggered ability on a permanent will each trigger separately.
- Flanking is a creature ability that triggers when the creature is blocked by one without flanking, giving the blocker -1/-1. Cavalry Master has flanking and says "Other creatures you control with flanking have flanking." If you have two Cavalry Masters and either one becomes blocked, it will trigger flanking twice - once for its own effect, and once again for the other Cavalry Master, for a net -2/-2 to its blockers.
- Genju of the Fields has an activated ability that turns an enchanted land into a creature and gives it "whenever this creature deals damage, you gain that much life." If this ability is activated three times, the enchanted land will be a creature with three instances of the life-gaining ability. If it then deals 2 damage, each of those three abilities will trigger, each will see that 2 damage was dealt, and each will give 2 damage to its controller, for a total gain of 6 life.
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