Explaining Priority, the Stack, and Layers

Jul
10

Explaining Priority, the Stack, and Layers

Magic: The Gathering is an extremely complex game, as anyone who has ever played can tell you. This complexity comes at the cost of having occasional extremely confusing rules and board states, especially since so many cards modify the rules of the game itself. In this article, I will attempt to briefly explain three key rules in Magic that, once understood, can determine the outcome of the vast majority of situations. Let’s start with the simplest and work our way up.

Priority

Priority, literally defined as “the right to take precedence or to proceed before others,” is the MtG rule explaining who has the ability to take action at any given moment. Priority is fairly simple to understand on the face of it: during each step of a turn, after any triggers that would occur at the beginning of that step have been placed on the stack (explained later), each player gets the chance to take an action (this may be casting a card or using an ability). Wizard of the Coast uses the terms AP (active player) and NAP (non-active player) when dealing with priority. In most situations, when a player would gain priority the AP gets priority first (the one exception being that after the NAP adds something to the stack that player retains priority). They may take any legal action they like, and when they are finished the priority passes to the NAP with all of the AP’s actions on the stack. It is very important to note that the AP can take multiple actions before passing priority. It’s usually a terrible idea to do this, but it is legal. In multiplayer games, priority works the same way: the AP gains priority first, followed by each NAP in turn order.

The Stack

The stack in MtG goes hand in hand with priority. Whenever anything is cast, triggered, or activated, it goes on the stack and the cycle of priority begins. Once both players pass priority without adding to the stack, the top action on the stack is resolved, and players gain priority again. The stack is really a very simple concept that confuses many new players to the game, but it’s very easy to visualize. When you are new to the game, you can use physical cards to represent the stack (spell casts can use the actual card, abilities can use a playing card or something, just remember which goes with which ability) and just lay the cards on top of each other as you add them to the stack. Then you resolve them by resolving the top card and going back down. Just remember that every time a card gets removed from the stack, the AP gains priority.

If multiple triggers would be added to the stack as the result of the same action, they are added in turn order (AP – NAP). If a single player controls multiple triggers, that player decides which order they go on the stack in.

Exception to the Stack: Combat Damage

I just said everything in MtG uses a stack didn’t I? Well, I lied. Combat damage determined at the end of the combat phase doesn’t. It all happens at the same time. However, there’s one very important thing to know about how damage works. Creatures do not die when they have lethal damage placed on them. They die as what’s called a state-based action, which happens any time a player would gain priority. Why does this matter? Well, at the start of this article I mentioned that at the start of each step a player only gains priority after any triggers go on the stack. Let’s look at a specific example of when creature death as a state-based action matters, involving the M14 card Archangel of Thune. In this example, you are attacking with the Archangel as well as a Grizzly Bears (which is unblocked). Your opponent elects to block the Archangel with his Serra Angel. Here’s what happens in this situation:

  1. Combat damage is assigned. 3 damage is placed on the Serra Angel, 4 on the Archangel of Thune, and 2 damage is dealt to your opponent. 3 life is gained with the Archangel’s lifelink (which is not a trigger – it happens at the same time as damage).
  2. Archangel of Thune’s trigger is placed on the stack because you gained life.
  3. The AP (you) gains priority with the Archangel’s trigger on the stack. State-based actions are now checked, and since the Archangel has damage on it equal to its toughness, it dies. However, that trigger is still on the stack!
  4. If both players pass priority, the trigger resolves. Your Bear gains +1/+1.

Without a full understanding of the stack and priority system, it’s easy to come to one of two incorrect conclusions here (either your Archangel becomes 5/5 and lives, or the bear stays 2/2). Following the stack and priority gets us to the correct conclusion to this situation.

Advanced Concept: Layers

Priority and the stack handle everything that is a one-time action: casting spells, activating abilities, or having abilities triggered. But how do cards that have persistent effects like Mutavault interact with other persistent effects? As a specific example, let’s look at the annoyingly complex interaction between Mutavault and Shapesharer. Let’s assume that one player controls both of these two cards and has earlier in the turn activated Mutavault’s ability to make it a 2/2 creature with all creature types. He then, thinking he’s awesome, activates Shapesharer targeting his opponent’s Kozilek, Butcher of Truth. This unfortunately doesn’t work quite as our player hopes (he doesn’t get a 12/12 creature), and I’ll demonstrate why with layers. The layers are as follows:

  1. Copy Effects
  2. Control-Changing Effects
  3. Text-Changing Effects
  4. Type-Changing Effects
  5. Color-Changing Effects
  6. Ability Adding/Removing Effects
  7. (A) Effects from Power/Toughness Defining Abilities (such as Tarmogoyf)
    (B) Power/Toughness Setting Effects
    (C) Power/Toughness Modifications from Effects
    (D) Power/Toughness Modifications from Counters
    (E) Effects that Switch Power/Toughness

Whew! Now how does this all work? When there are multiple continuous effects on the board, they all take effect in the order above. If there are multiple effects in the same category, they take effect in the order they were played originally, so effects that have been on the board longer take effect first.

How does this work with our example above? In Layer 1, the copy affect takes place. The Mutavault is now Kozilek, Butcher of Truth – with that card’s name, abilities, types, power, toughness, and so on and so forth. Layers 2 and 3 have nothing using them, so we move to Layer 4. Here, the Mutavault (which is now actually Kozilek) becomes a Land Creature with all creature subtypes through its ability. Layers 5 and 6 have nothing in this situation, so we move up to layer 7B. Here, Mutavault’s effect takes place making it a 2/2. After all of this insanity, our hero can attack with a card that reads:

“Kozilek, Butcher of Truth.      12

Land Creature – All Creature Types.

When you cast Kozilek, Butcher of Truth, draw four cards.
Annihilator 4
When Kozilek, Butcher of Truth is put into a graveyard from anywhere, its owner shuffles his or her graveyard into his or her library.

2/2”

So, our player DOES get to attack with something with annihilator 4, but it’s only a 2/2. As a side note, since copy and creature effects can’t exist in any zone other than the battlefield, if something kills this 2/2 in combat it will not get a chance to shuffle the graveyard into the library. Kozilek isn’t put into the graveyard, just a sad Mutavault that’s only a Land at that point.

Conclusions

I hope this article was useful! All of these rules can be complex, but they are all logical and will arrive at the correct answer if you just follow the steps. If you’re a new player and aren’t sure how something works, just ask!

About Andy Rauff

New to the Magic: The Gathering scene but an old hand at TCGs, Andy is looking forward to writing some MtG articles in the hopes they'll be useful for others getting started with the game like himself. By day, Andy works as a pianist, organist, and freelance web developer.

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