A.4.1- A triggered ability begins with the word "when", "whenever",
or "at". The phrase starting with one of these words is the trigger
condition. [CompRules 1999/04/23] More completely, a triggered ability
reads "When/whenever/at
A.4.2- Triggered abilities check for their condition at all times, even during the middle of an event (see Rule T.1.6). [D'Angelo 1999/05/01]
A.4.3- Triggered abilities are not played. They wait until the next time a player receives priority, then they are put on the stack (see Rule T.2). It is placed on as a pseudospell, with a copy of its source's text, color, and controller at the time it triggered. [CompRules 1999/04/23]
A.4.4- Triggered abilities only trigger while the permanent is in play unless otherwise stated. See Rule A.1.3. [CompRules 1999/04/23]
A.4.5- Once triggered, the ability exists independently of the card or token which generated it. Destruction or removal of the source after that time will not affect the ability. [CompRules 1999/04/23]
A.4.6- If the ability says that its source does something, then the characteristics of the source at the time the ability resolves are used. [CompRules 1999/04/23] If the source is no longer in play, its last known characteristics are used. [CompRules 1999/04/23]
A.4.7- You follow the rules for announcing a spell or ability at the time the ability is put on the stack. This means making targeting choices and the choice of mode as per Rule T.4.2. [CompRules 1999/04/23]
A.4.8- If multiple triggered abilities are being placed on the stack at one time, ones controlled by the current player go on the stack first, in any order that player chooses. Then ones controlled by the other player, in any order that player chooses. [CompRules 1999/04/23]
A.4.9- The controller of a triggered ability is the controller of the permanent which generated it (if it is from a permanent) or the controller of the effect which generated it. The controller is set at the time the ability triggers, not when it is put on the stack. [CompRules 1999/11/01]
A.4.10- This rule has been removed. [Rules Team 2001/05/01] Previously, a triggered ability's controller could be different from the permanent's controller if the other player got a choice.
A.4.11- This rule has been removed. [Rules Team 2001/05/01] Previously, if a triggered ability offered both players a choice, it put separate abilities on the stack for each player. Now only one ability is placed on the stack and is resolved for all players.
A.4.12- A triggered ability triggers once for each time its trigger condition occurs. It can trigger more than once on a single event in the game if the condition occurs more than once. [CompRules 1999/04/23] For example, if an ability reads "Whenever a land is put into a graveyard from play, ..." then the ability would trigger 5 times if 5 lands were put into the graveyard at one time.
A.4.13- Some triggered abilities give a player a choice of doing something
or doing nothing. These use the word "may" and are worded
like "When
A.4.14- A triggered ability may include a test in its effect, worded
as "When/whenever/at
A.4.15- A triggered ability that watches for a game state checks its condition at all times, even during the middle of the resolution of a spell or ability. This is called a "state-based trigger". [CompRules 1999/04/23]
A.4.16- A triggered ability that watches for a game state will not trigger again until any unresolved pseudospell placed on the stack by the ability resolves or is countered. For example, an ability reads "When your hand is empty, draw a card." If its controller plays the last spell from their hand, it triggers once and won't trigger again until after this resolves, and then you empty your hand again. [CompRules 1999/04/23]
A.4.17- An ability that reads "Whenever
A.4.18- An ability that reads "Whenever
A.4.19- A triggered ability that triggers on a zone change (such as going to the graveyard) and which tries to do something to the card will look for the card in the zone it went to. If the card leaves the zone before the triggered ability resolves, the ability will fail to do anything. [CompRules 1999/04/23] For example, "When this card goes to the graveyard from play, remove it from the game" will trigger when the card goes to the graveyard. If the card is not still in the graveyard on resolution, the ability will not do anything.
A.4.20- "Leaves play" triggers are handled with a special rule. Since the permanent will no longer be in play, if the ability wants to check anything about the permanent or game state, the ability "looks back in time" to just before the permanent left play. Any continuous effects or triggered abilities present at that time apply. [CompRules 1999/04/23] For example, two creatures are in play with an artifact that says "Whenever a creature is put into a graveyard from play, you gain 1 life". A spell is played that destroys all artifacts, creatures, and enchantments. The artifact's ability triggers twice even though the artifact went to the graveyard at the same time as the creatures. If you back up time to right before anything left play, it was there. [CompRules 1999/04/23]
A.4.21- "Comes into play" triggers see the permanent after any continuous effects are applied. For example, if Blood Moon is in play then a non-basic land will enter play as a Mountain and something triggering off a Mountain coming into play would trigger but something triggering off a non-basic land coming into play would not. [D'Angelo 1999/05/01]
A.4.22- Effects that prevent abilities from being played do not prevent triggered abilities from being put on the stack. [CompRules 1999/04/23]
A.4.23- An ability that triggers whenever a creature is blocked by a specific number of creatures can trigger after the declaration of blockers if a spell or ability adds an additional blocker. For example, Johtull Wurm's ability triggers when blocked by two or more creatures. If one blocked it during declaration of blockers, then another was added later, the ability will trigger. Note that swapping one blocker for another one blocker will not cause such abilities to trigger. [WotC Rules Team 2000/02/16] Note that such an ability can trigger only once per combat phase. [D'Angelo 2000/02/25]
A.4.Ruling.1- Because triggered abilities are placed on the stack (see Rule T.2), triggered abilities can become nested. For example, if one spell's resolution caused two triggered abilities to trigger, then during the resolution of the first one of these triggered abilities, another one is triggered. The newly triggered ability is resolved first since it will be on top of the stack. [D'Angelo 1999/05/01]
A.4.Ruling.2- Abilities which trigger on you losing control of something also trigger if that something leaves play since you lose control of it when it leaves play. [bethmo 1996/01/15]
A.4.Ruling.3- Multiple abilities can trigger off the same thing happening, but a single permanent's ability will never double-fire off of a single action. For example, if you have two Scavenging Ghouls in play, each will get a counter if a creature dies, but neither will get two counters. [D'Angelo 1999/05/01]
A.4.Ruling.4- If the trigger condition is prevented or replaced, then the triggered ability will not trigger. [CompRules 1999/04/23]
A.4.Ruling.5- The text "This permanent comes into play as/with
A.4.Ruling.6- A triggered ability can trigger on its own permanent leaving play. See Rule A.4.20. [D'Angelo 1999/05/01] For example, an Animate Artifact on a Soul Net will allow you to use the Soul Net's ability when it is killed.
A.4.Ruling.7- Rule A.4.20 only applies to determine stuff read in by the trigger. If the trigger sets up a continuous effect with an end condition, the end condition is checked against the current game state, not the state before the permanent left play. [D'Angelo 1999/05/26]
A.4.Ruling.8- If a card reads like "Whenever a player plays a