From the Comprehensive Rules (Khans of Tarkir)
- 611. Continuous Effects
- 611.1. A continuous effect modifies characteristics of objects, modifies control of objects, or affects players or the rules of the game, for a fixed or indefinite period.
- 611.2. A continuous effect may be generated by the resolution of a spell or ability.
- 611.2a A continuous effect generated by the resolution of a spell or ability lasts as long as stated by the spell or ability creating it (such as "until end of turn"). If no duration is stated, it lasts until the end of the game.
- 611.2a Some continuous effects generated by the resolution of a spell or ability have durations worded "for as long as...." If the "for as long as" duration never starts, or it ends before the moment the effect would first be applied, the effect does nothing. It doesn’t start and immediately stop again, and it doesn’t last forever.
Example: Master Thief has the ability "When Master Thief enters the battlefield, gain control of target artifact for as long as you control Master Thief." If you lose control of Master Thief before the ability resolves, it does nothing, because its duration——as long as you control Master Thief——was over before the effect began.
- 611.2c If a continuous effect generated by the resolution of a spell or ability modifies the characteristics or changes the controller of any objects, the set of objects it affects is determined when that continuous effect begins. After that point, the set won’t change. (Note that this works differently than a continuous effect from a static ability.) A continuous effect generated by the resolution of a spell or ability that doesn’t modify the characteristics or change the controller of any objects modifies the rules of the game, so it can affect objects that weren’t affected when that continuous effect began.
Example: An effect that reads "All white creatures get +1/+1 until end of turn" gives the bonus to all permanents that are white creatures when the spell or ability resolves——even if they change color later——and doesn’t affect those that enter the battlefield or turn white afterward.
Example: An effect that reads "Prevent all damage creatures would deal this turn" doesn’t modify any object’s characteristics, so it’s modifying the rules of the game. That means the effect will apply even to damage from creatures that weren’t on the battlefield when the continuous effect began. It also affects damage from permanents that become creatures later in the turn.
- 611.2d If a resolving spell or ability that creates a continuous effect contains a variable such as X, the value of that variable is determined only once, on resolution. See rule 608.2g.
- 611.2e If a resolving spell or ability both puts a nontoken permanent onto the battlefield and creates a continuous effect stating that the permanent "is [characteristic]," that continuous effect applies simultaneously with the permanent entering the battlefield. This characteristic is usually a color or a creature type. If the continuous effect says the permanent "becomes [characteristic]" or "gains [an ability]," that effect applies after the permanent is on the battlefield.
Example: Arbiter of the Ideal puts an artifact, creature, or land card onto the battlefield and says, in part, "That permanent is an enchantment in addition to its other types." An ability that triggers whenever an enchantment enters the battlefield would trigger. The permanent doesn't enter the battlefield and then become an enchantment.
- 611.3. A continuous effect may be generated by the static ability of an object.
- 611.3a A continuous effect generated by a static ability isn’t "locked in"; it applies at any given moment to whatever its text indicates.
- 611.3b The effect applies at all times that the permanent generating it is on the battlefield or the object generating it is in the appropriate zone.
Example: A permanent with the static ability "All white creatures get +1/+1" generates an effect that continuously gives +1/+1 to each white creature on the battlefield. If a creature becomes white, it gets this bonus; a creature that stops being white loses it.
- 611.3c Continuous effects that modify characteristics of permanents do so simultaneously with the permanent entering the battlefield. They don’t wait until the permanent is on the battlefield and then change it. Because such effects apply as the permanent enters the battlefield, they are applied before determining whether the permanent will cause an ability to trigger when it enters the battlefield.
Example: A permanent with the static ability "All white creatures get +1/+1" is on the battlefield. A creature spell that would normally create a 1/1 white creature instead creates a 2/2 white creature. The creature doesn’t enter the battlefield as 1/1 and then change to 2/2.