Rule 613.4

The application of continuous effects as described by the layer system is continually and automatically performed by the game. All resulting changes to an object's characteristics are instantaneous. Example: Crusade is an enchantment that reads "White creatures get +1/+1." Crusade and a 2/2 black creature are on the battlefield. If an effect then turns the creature white (layer 5), it gets +1/+1 from Crusade (layer 7c), becoming 3/3. If the creature's color is later changed to red (layer 5), Crusade's effect stops applying to it, and it will return to being 2/2. Example: Gray Ogre, a 2/2 creature, is on the battlefield. An effect puts a +1/+1 counter on it (layer 7d), making it 3/3. A spell targeting it that says "Target creature gets +4/+4 until end of turn" resolves (layer 7c), making it 7/7. An enchantment that says "Creatures you control get +0/+2" enters the battlefield (layer 7c), making it 7/9. An effect that says "Target creature becomes 0/1 until end of turn" is applied to it (layer 7b), making it 5/8 (0/1, plus +4/+4 from the resolved spell, plus +0/+2 from the enchantment, plus +1/+1 from the counter).

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