Q: Could you clarify the interactions that exist between effects that force a creature's P/T to be specific values versus effects that raise, lower, or switch that creatures P/T?
A: From the Magic Rules Corner:
With Lignify, Godhead of Awe, Snakeform, the Eventide Mimics, and more arriving just in the last year, variations on this question have come up a lot. Rather than answer them piecemeal, we decided to bundle them together and teach you the underlying structure that helps determine the answers. We'll give a few examples at the end.
Rule 418.5 in the Comprehensive Rules covers the interaction of continuous effects—in other words, it tells you how to determine what characteristics a creature or other object in the game ends up with when multiple continuous effects apply to it at once. This is accomplished through a series of layers, which determine the order in which to apply those continuous effects to find the end result.
The first question, then, is "What is a continuous effect?" It's an effect from a spell or ability that lasts for some actual length of time. Giant Growth creates a continuous effect, because its effect lasts until the end of the turn. Each of Figure of Destiny's abilities creates a continuous effect, because its effect lasts until the end of the game. Glorious Anthem's ability creates a continuous effect, because its effect is true for as long as Glorious Anthem is in play. (One-shot effects, on the other hand, happen immediately and then have no lasting effect. "Draw three cards" is an example of a one-shot effect.)
Understanding how the game handles continuous effects can be a mind-bending concept—but the key is right there in the name, and in the difference between continuous effects and one-shot effects. A one-shot effect (from Harmonize or Terror, for example) changes the game state then immediately dissipates. A continuous effect, on the other hand, hangs around. Giant Growth might feel like a one-shot effect because it's an instant, but that's not how it behaves. It doesn't deposit a +3/+3 bump on a creature and go away. Rather, imagine that its effect invisibly hovers over that creature for the stated period of time. As long as it's hovering, the +3/+3 bump is there. The tricky thing is that all continuous effects hover like this. If a creature is affected by Mirrorweave, Mind Bend, Lignify, Night of Souls' Betrayal, and Giant Growth, that creature has a lot of stuff hovering over it! The layer system determines what order those effects are applied in. Importantly, the order in which they're applied is not necessarily the order in which they showed up!
So let's take a look at the layers. There are six of them, and they're applied in order from 1 to 6. In every moment of every game, the game automatically rebuilds each permanent from the ground up. It starts with the printed wording on a card (or the original characteristics of a token), then applies all the continuous effects hovering over it in layer order, and winds up with the permanent's current characteristics. It's an eternal refresh.
The unofficial Layer 0 is the original card or token. That's what you start with.
Layer 1 is for copy effects. (Mirrorweave, Clone, and so on.) Those come first.
Layer 2 is for control-change effects. (Threaten, Confiscate, and so on.)
Layer 3 is for text-changing effects. (Glamerdye, Artificial Evolution, and so on.)
Layer 4 is for type-changing effects. (Imagecrafter, Leyline of Singularity, and so on.)
Layer 5 is a catchall; it's for everything else except power and toughness-changing effects. Generally, this means things that change a permanent's colors and abilities (Aphotic Wisps, Canopy Claws, and so on.)
Layer 6 is for power- and toughness-changing effects. But there's more to this story...
We'll get to layer 6 in a moment. First, you may have noticed that lots of spells and abilities have effects that fit into multiple layers. In those cases, the effects are split apart and each piece applies in the appropriate layer. For example, Treetop Village's effect says that it becomes a creature that's still a land (layer 4), becomes an Ape (layer 4), becomes green (layer 5), has trample (layer 5), and becomes 3/3 (layer 6)!
OK, back to layer 6. There are lots of different kinds of effects that can change a creature's power and toughness, so layer 6 has a layer system all its own. There are five sublayers, and they're applied in order from a to e.
Sublayer 6a is for characteristic-defining abilities (or CDAs).
If a creature has a * in its power/toughness box, it's got a CDA to define its power, toughness, or both. This comes first because it's a substitute for a number in the power/toughness box. Examples include Overbeing of Myth, Coiling Woodworm, and Tarmogoyf.
Sublayer 6b is a catchall; it's for everything not covered in sublayers 6a, 6c, 6d, or 6e.
Specifically, this covers effects from resolved spells and abilities that change a creature's power or toughness (like Giant Growth or Afflict), effects from resolved spells and abilities that set a creature's power or toughness to a specific value (like Battlegate Mimic or Snakeform), and effects from static abilities that set a creature's power or toughness to a specific value (like Lignify or Godhead of Awe).
Sublayer 6c is for changes from counters.
These are effects from +1/+1 counters, -1/-1 counters, and any other counters that directly alter power and/or toughness. Applying them in this sublayer means they always apply.
Sublayer 6d is for effects from static abilities that modify power or toughness but don't set them to a specific value.
These are effects that, like counters, are sitting on the board giving a constant power/toughness bonus (or a constant negative). Examples include Glorious Anthem, Night of Souls' Betrayal, Blanchwood Armor, and Vulshok Battlegear. Applying them in this sublayer means they always apply.
Sublayer 6e is for effects that switch a creature's power and toughness.
These are always applied last. That makes the bookkeeping easier; you can do all of the other calculations first, then switch the resulting values. Examples include Inside Out and Crag Puca.
Effects that apply in sublayer 6c, 6d, and 6e never conflict with one another—they all apply. But effects in sublayer 6b can conflict with one another, and that's where the confusion comes in. If effects within a particular layer or sublayer overwrite one another (for example, one effect says a permanent is green then a later one says it's blue, or one effect gives a creature +3/+3 then a later one says it becomes 1/1), the effects are usually applied in timestamp order. This is a fancy way of saying that they're applied in the order that they first took effect, so the most recently played one wins. There are exceptions, but they're obscure enough to not worry about.
Layer 6 in Action
Shorecrasher Mimic (2/1) is in play. Its controller plays a spell that's green and blue. Shorecrasher Mimic's ability triggers and resolves, and it becomes 5/3 (layer 6b). Later in the turn, it's targeted by Giant Growth (also layer 6b). The effects are applied in timestamp order, so Shorecrasher Mimic is 8/6.
Shorecrasher Mimic (2/1) is targeted by Giant Growth (layer 6b), giving it +3/+3, making it 5/4. Later in the turn, its controller plays a spell that's green and blue. Shorecrasher Mimic's ability triggers and resolves (also layer 6b). The effects are applied in timestamp order, so Shorecrasher Mimic is 5/3.
Hill Giant (3/3) is enchanted with Moldervine Cloak (layer 6d). It's 6/6. It's then targeted by Snakeform, setting its power and toughness to 1/1 (layer 6b). Hill Giant is 4/4.
Example 4: Hill Giant (3/3) has two -1/-1 counters on it (layer 6c). It's 1/1. Godhead of Awe comes into play, setting all creatures' power and toughness to 1/1 (layer 6b). Hill Giant is -1/-1. Its toughness is less than 1, so it's put into its owner's graveyard.
Giant Spider (2/4) is targeted by Inside Out (layer 6e). It's 4/2. Later in the turn, it's enchanted with Torpor Dust (layer 6d). Giant Spider is 4/-1. Its toughness is less than 1, so it's put into its owner's graveyard.
Overbeing of Myth is in play. Its controller has five cards in hand, so it's 5/5 (layer 6a). An effect puts two +1/+1 counters on it (layer 6c), making it 7/7. It's targeted with Giant Growth (layer 6b), making it 10/10. Later in the turn, it's targeted by Snakeform (layer 6b). Snakeform applies after the Overbeing's CDA (because of sublayer order) and after Giant Growth's effect (because of timestamp order), but before the effect from the two counters (because of sublayer order again). Overbeing of Myth is 3/3.
Giant Spider (2/4) is targeted by Giant Growth (layer 6b). It's 5/7. Mirrorweave then turns it into a copy of Wood Elves (layer 1). It's now a 4/4 Wood Elves.
So next time you play Wings of Velis Vel on an activated Marsh Flitter with two +1/+1 counters on it, you'll know what happens!
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