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When does priority actually matter in magic? I have been playing Magic since Time Spiral and have never once seen the rules for priority matter. I've never heard a question from a player about a game situation to which the answer was, "Oh, well it's right here in the rules for priority."

Let me elaborate. Most other games handle questions of precedence (I want to do this but my opponent wants to do something conflicting) with a system of priority. Magic handles questions of this type with the stack. Here are some examples:

  • What tap effect has precedence?

  • I have a 2/2 and I want to play Giant Growth on it while my opponent wants to kill it with Lightning Bolt? Resolved by the stack. Priority doesn't actually matter here. It's not a question of who has the option to act first, it's about who does act first, and because the person that acts first in this situation loses, the solution (given both players run Telepathy) is for both players to pass and the game to move on to the next step or phase. But this isn't a quirk of priority, this is that the Nash Equilibrium given perfect information and how the stack works is for neither player to cast his/her spell.

  • I want to cast Silence on my opponent during his/her upkeep but he/she wants to cast Lightning Bolt on me first. Sure, a player get's priority on his/her turn before other players, so he/she has the option to cast a spell before I can cast Silence, but it doesn't matter! Since this all happens during the upkeep, the spell I want to cast has to be instant speed anyways, so it can be cast in response to Silence while Silence is on the stack. Since Silence only takes effect on resolution, what order it and another spell are cast in won't change either spell's resolution (unless the other spell is a counterspell). In almost all situations, Silence only changes the ability for the player to cast things post-upkeep.

In all of the above situations, the stack is what determines what happens. Players priority in putting things on the stack doesn't matter since in the above cases (and the overwhelming majority of cases I've seen), as it is the ability to react that matters, not the ability to act first.

So, given how the stack works, does priority actually matter in Magic? If so, what types of situations do the rules for priority needed to actually determine what happens?


For reference, here are the rules for priority:

  • 116.3. Which player has priority is determined by the following rules:
    • 116.3a The active player receives priority at the beginning of most steps and phases, after any turn-based actions (such as drawing a card during the draw step; see rule 703) have been dealt with and abilities that trigger at the beginning of that phase or step have been put on the stack. No player receives priority during the untap step. Players usually don't get priority during the cleanup step (see rule 514.3).
    • 116.3b The active player receives priority after a spell or ability (other than a mana ability) resolves.
    • 116.3c If a player has priority when he or she casts a spell, activates an ability, or takes a special action, that player receives priority afterward.
    • 116.3d If a player has priority and chooses not to take any actions, that player passes. If any mana is in that player's mana pool, he or she announces what mana is there. Then the next player in turn order receives priority.
  • 116.4. If all players pass in succession (that is, if all players pass without taking any actions in between passing), the spell or ability on top of the stack resolves or, if the stack is empty, the phase or step ends.
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    The point I'm trying to make is that it's a good example of how the stack works, and that priority only doesn't matter because Telepathy was added to the mix. On each side, optimal play is different if the players don't have perfect information. And, if you're the active player and you pass priority, your opponent can pass priority and you miss the opportunity to cast Giant Growth because the step ends. – murgatroid99 Nov 25 '14 at 22:14
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    A common real life example: It practically guarantees a use of your Planewalker's loyalty ability before your opponent can kill it. See here for details. – ikegami Nov 25 '14 at 22:18
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    @Zags I am questioning the validity of a statement you made in your question. I'm not answering the question in these comments, and putting these comments in an answer would be improper. – murgatroid99 Nov 25 '14 at 22:20
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    Declining to act is called "passing priority". You can't decline to "use priority", any more than you can decline to use the stack, or phases. If that's actually what you meant, then you seem to be misunderstanding something important about priority. And finding out what exactly the asker understands about their question is one of the most important use of comments. – murgatroid99 Nov 25 '14 at 22:46
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    You're making it even more complicated, and you still haven't answered my question: you seem to be misunderstanding how priority works at some basic level, and I'm trying to understand how you think it works. And, in that light, the completeness of your answer is suspect. – murgatroid99 Nov 25 '14 at 23:03
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Priority can matter, but only in rare corner cases. Here are the main types of situations where priority matters. Some of these are pretty obscure.


Split second cards

It's your turn. You have in hand Dark Depths and Vampire Hexmage for the infamous Marit Lage combo. Your opponent has Sudden Death in hand. Can you summon Marit Lage? The answer is yes. You retain priority through playing Dark Depths and casting Vampire Hexmage, then your opponent gains priority while Hexmage is on the stack (which is still too early to cast Sudden Death on it), but then your opponent cannot respond again between the Hexmage resolving, entering the battlefield, and you activating its sacrifice ability. As the active player, you can retain control through this entire process. That said, if you were putting the Hexmage onto the battlefield on your opponent's turn through Aether Vial, they would be able to Sudden Death it before you could use its ability because they have priority.

If instead your plan is to win via Chance Encounter + Frenetic Efreet, you can still do it facing a Sudden Death if it is your turn and you play Chance Encounter first. After the Efreet enters the battlefield, you have priority and retain it through as many activations of its ability as you want. You can have a billion activations of its ability on the stack by the time your opponent has the option to Sudden Death it, giving you near statistical certainty that you will have enough counters on Chance Encounter to win.

The reason split second cards matter with priority is that they temporarily change how the stack works. Instead of a player being able to respond at any time, they can't respond until the split second card has finished resolving, meaning when you can cast them matters since players cannot react with other spells or abilities once they are cast.


Sorcery speed abilities (such as planeswalker abilities)

When a planeswalker spell resolves, assuming it was the bottom spell on the stack and cast by the active player (so not cast using Vedalken Orrery or the like), the active player has the option to activate one of it's abilities before anyone else can do anything. This is important because if someone else could do something, they could Lightning Bolt your planeswalker before you could use one of its abilities. This pretty much only matters with planeswalkers because their abilities are sorcery speed. Almost all other activated abilities of permanents are instant speed. Those that aren't also benefit from priority. This is anything with the text "Activate this ability only any time you could cast a sorcery." such as Aggravated Assault. Basically, if you have a permanent enter the battlefield under your control, you can put one of its sorcery speed abilities on the stack before anyone else can kill or destroy the permanent.

The exception to this is if the card with a sorcery speed ability triggers an enter the battlefield ability, in which case the stack will not be empty when you gain priority and players will be able to use instant speed removal before you can use its abilitiy.

Also in this category are the ability to trigger abilities with sorcery speed cards, such as the situation in this question. If the active player has just cast Kor Spiritdancer and it resolves and doesn't trigger any enter the battlefield abilities, the active player will be able to cast an Aura and consequently trigger the Spiritdancer's ability before the other player has the chance to play instant speed removal.


A change in casting eligibility as a result of a non-stack action (such as paying a cost or a mana ability)

You are playing a mill deck against a burn deck. Your opponent has Searing Blaze in hand and you are at 1 life. Meanwhile, your opponent has three cards left in his/her library, and they are all copies of Lightning Bolt. You have Altar of Dementia on the battlefield and a beefy creature in hand. Who wins?

This matters includes, but also goes far beyond, priority. First, Searing Blaze requires that you have a creature on the battlefield for it to be cast. The requirement is just for casting; the creature need not be there on resolution since as long as any of a spell's targets are still legal, the spell will resolve. Additionally, sacrificing a creature to the Altar is part of the cost for the altar, and so the sacrifice doesn't use the stack. Consequently, there is an action that doesn't use the stack that changes whether or not another action can be put on the stack. It's a fairly rare situation, but it is one where priority matters. If it is your turn, your creature can enter battlefield and then be sacrificed to the altar without your opponent getting priority. By the time he/she has priority, the creature is already in the graveyard, and so Searing Blaze cannot be cast.


Retaining priority after playing a land

This is also going to be a bit contrived. You have some creature that you need to give double strike in order for you to win. You have Assault Strobe and a Mountain in hand, and several Mountains on the battlefield. Your opponent has Mana Short. Because of priority, they cannot stop you from casting Assault Strobe. If they cast Mana Short before your main phase, you can just play the Mountain on your main phase and then cast Assault Strobe with it. Meanwhile, if they wait, you retain priority after playing the land, and so can still cast Assault Strobe before they cast Mana Short. The first chance they have to cast Mana Short during your main phase is in response to Assault Strobe. Basically, this just means that Mana Short should be played before your opponent's main phase and that it doesn't deprive them of mana sources they get after that point, but this isn't exactly news.

As is the case for the sorcery speed abilities, if the land entering the battlefield triggers an ability, the stack won't be empty when you receive priority, so your opponent will have a chance to use Mana Short before you can cast Assault Strobe since sorceries can only be cast when the stack is empty.


Breaking the symmetry of hidden information

This is one introduced in comments and is perhaps the hardest to formalize. Take the example in the question where a player is attacking with a 2/2 and has Giant Growth while the defender has Lightning Bolt. It is now the declare blockers step, the last time to cast spells before combat damage. If neither player knows about the other's threatened instant, than each player is eager to cast his/her card but whoever does so first loses the interaction. Here, that the active player must chose first is a disadvantage because it causes the active player to be the one to make the bad decision rather than the non-active player.

That said, the application of priority rules here is only relevant if players are using a low degree of short-cutting (namely the non-active player waits for explicit verbal confirmation that the active player is not acting during the declare blockers step before announcing "I do this during the declare blockers"). In addition, no player trying to win will invoke priority rules here if he or she is erroneously skipped; the ability to respond is far more valuable. As a final point to why this situation is particularly rare, both players must have to have basically equal willingness to take an action that is disadvantageous given full information. For this to actually be the case requires fairly specific setups of both game board and player psyche, and occurs rather rarely in practice.


Multiplayer

If multiple players want to respond to a particular event (such as by countering a mass-removal spell), whoever is earlier in priority will be at the disadvantage of acting first. Perfect information in this situation doesn't help. The first player with the option to counter the spell cannot know for certain the true priorities of the next player with the option to counter it. The first player must chose whether to counter the spell or risk it going uncountered where the second player has the leisure of only needing to counter it if the first does not.

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    @diego – Actually, 116.3b handles the case for priority after a spell or ability resolves (giving it to the active player). 116.3c is for responding to casting your own spell or activating an ability. – Hao Ye Nov 25 '14 at 21:29
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    I think you might've missed possibly the most common time it matters: combat tricks. For example, say you attack with a creature and want to cast Giant Growth on it to help finish off your opponent, but are worried they might have a Doom Blade. You're the active player, so you get priority first, and have to decide whether to cast your spell without knowing whether the creature is going to die, while your opponent can sit back and wait to see if you pump it before they spend their removal spell. – Cascabel Nov 25 '14 at 21:58
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    @Zags Nope, it matters. Yes, they can kill it either way. But because you (the attacking player) get priority first, they can also wait to see that you haven't pumped it, then decide not to kill it, and then you won't get another chance to pump it. – Cascabel Nov 25 '14 at 22:07
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    That situation can actually be further generalized. Because of the priority system, the active player gets first choice of what to do, but only the non-active player can respond to their opponent choosing to do nothing (replace active player for whoever the rules say gets priority first if the stack is non-empty). – murgatroid99 Nov 25 '14 at 22:09
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    About the rare situation with bolt and giant growth: that sort of thing is probably much more common for those of us who play on MTGO, because there are no shortcuts there and everything relating to priority is very explicit. – RemcoGerlich Nov 26 '14 at 9:24
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Preface

While the answer by the OP is a pretty good list of examples when priority matters, the question as posed is more about "Stack vs Priority", as in, "Do the rules need the priority system when it already has the Stack?". A philosophical question warrants a philosophical answer. While such an answer is buried inside the comments, chat, and OP's answer, in my opinion it deserves its own answer.


The Stack vs the priority system

And this answer is: Yes, both the Stack and the priority system are important because they are solutions to different problems.

The rules for the Stack and for priority are so entwined with one another that it is difficult to tell them apart unless you look carefully at the rules. And carefully we shall look.


The Stack

I said the Stack and the priority system are solutions to different problems, so let's look at the problems that the Stack is trying to solve:

  1. How do the rules handle players responding to spells and abilities? They must do that if spells and abilities that target other spells/abilities (e.g. counterspells) are to work properly. If players are to be able to respond to spells and abilities, they can neither resolve immediately nor go into a limbo when they are cast/activated/triggered.
  2. In what order do the effects of different spells and abilities happen?

The rules for the Stack[405] solve them by saying:

  1. When spells and abilities are cast/activated/triggered, they are put on the Stack.[405.1] The Stack is the holding area for spells and abilities waiting to resolve.
  2. The Stack keeps track of the order in which objects are put there[405.2], and defines what happens if multiple objects are put there simultaneously[405.3].
  3. The objects on the Stack resolve in reverse order they are put there (LIFO).[405.5]

Now, it is not difficult to see a gaping hole in this solution: it defines the order in which spells and abilities resolve given the order they are added to the Stack, but says nothing about the latter.

Moreover, there are actions that do not use the Stack [405.6]. Naturally, the rules for the Stack do not govern them. So in what order do those happen? The previous paragraph is simply a special case of this since the act of adding objects to the Stack itself does not (and cannot) use the Stack.

This is where priority comes in.


Priority

As we did with the Stack, let's begin with the problems:

  1. When more than one player wants to be proactive (they all want to act first), who can act first?
  2. When all players want to be reactive (they all want to act last), who must act first?

The rules for priority[116] solve them by:

  1. Making so that, at any point in time, only at most one player can act.[116.1]
  2. Defining who that player is at any point in time.[116.3]
  3. Defining when spells and abilities resolve, and when steps and phases end[116.4], forcing players to act if they want to do so in response to a particular spell/ability or within a particular step/phase.

Since only at most one player can perform actions that do not use the Stack at any point in time, the order in which those actions happen is well-defined. If multiple players want to do so, those actions happen in the same order as those players' chance to act. If a single player wants to do so multiple times, that player chooses which action to perform first.


Examples

Now that we have differentiated the Stack rules and the priority rules, let's look at some examples.

Loyalty abilities

AP's Liliana, the Last Hope just resolved, emptying the Stack. NAP has a Lightning Bolt in hand.

The crux of this situation is that AP activating Lili's +1 puts a loyalty counter on Lili and puts it out of range of Bolt, but NAP casting Bolt prevents AP from activating Lili's +1. Since both actions do not use the Stack, and both players want to act first, the priority rules apply. AP can act first.

Split Second

This is essentially the same problem: a player taking an action that does not use the Stack (casting a Split Second spell) restricts what non-Stack action (casting/activating other spells/abilities) other players can take, resulting in both players wanting to act first.

Tap effect vs tap cost

AP has Wellwisher. NAP has Ballynock Trapper and wants to prevent AP from activating Wellwisher.

This is more difficult to analyze, since tapping for Wellwisher's ability does not use the Stack and Ballynock Trapper tapping Wellwisher uses the Stack.

Long story short, if AP actively wants to activate Wellwisher's ability, he can do so before NAP activate Trapper. That's priority. On the other hand, if AP wants to activate Wellwisher's ability only if NAP tries to prevent them from doing so using Trapper, then he can respond to Trapper's ability. That's the Stack.

Giant Growth vs Lightning Bolt

AP is attacking with a 1/1 with NAP at 2 life. NAP declares no blockers and it is currently in the Declare Blockers Step. Through previous events, both players know AP's only card in hand is Giant Growth and NAP's only card in hand is Lightning Bolt.

In this case, NAP has the advantage. If AP casts Growth, NAP can respond with Bolt to kill the 1/1. If AP does not cast Growth, NAP simply passes and take 1 combat damage from the 1/1.

Now, NAP's advantage does not come from their ability to respond to AP's spell because that advantage is symmetric -- AP can also respond to NAP's spell. Rather, NAP's advantage stems from NAP's ability to make their decision based on the other player's decision and AP's inability to do so. That, in turn, is due to AP receiving priority first in the Declare Blockers Step and thus having to make a decision first.

With that in mind, it does not even matter if NAP knows AP's hand. In NAP's perspective, as long as AP does not cast a pump spell, the reason why they do not is irrelevant. AP has decided to do nothing else and is bound to that decision if NAP decides the same. This is, again, by virtue of NAP receiving priority last.

To hit home the point, imagine if NAP is at 1 life instead of 2, then AP has the advantage instead. In this situation, if NAP passes, they die to combat damage. If NAP casts Lightning Bolt, AP can respond with Growth, saving the attacker. Here, AP's ability to respond prevails. The tables are turned because now the NAP wants to be proactive rather than reactive. The NAP is forced to cast Bolt regardless of AP's decision and as a result loses the advantage. On the other hand, AP now has the ability to make a decision based on NAP's decision to cast Bolt. Thus, the AP, instead of the NAP, has the advantage here.

  • If you'll let me know when you are done making edits, I will help you fix the subject-verb agreements in the post. – Rainbolt Nov 21 '16 at 15:25
  • I definitely agree that priority fills a gap that would otherwise exist in the rules; the question was really about where that gap is in game play. I like your philosophical distinction. However, I don't agree that your giant growth vs lightning bolt is a consequence of priority. The two sides of your example (the 1 life vs 2 life) illustrate that it is not priority that forces a player to act first there, but rather whichever player has a greater desire to avoid the default result of the situation. – Zags Nov 21 '16 at 20:15
  • I want to note that the giant growth vs lightning bolt is a very interesting question, requiring game theory and modeling asymmetric information to fully answer, and I believe that the application of priority falls under this heading (as I tried to address in my answer) rather than it being simply a consequence of priority and the stack. This is in large part due to there being a potentially different result that has nothing to do with priority if players have perfect information. – Zags Nov 21 '16 at 20:16

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