APNAP

Mystical Tutor card art

I mentioned Active Player, Non-Active Player (APNAP) in the last post. This post explains that concept, along with the associated concept of priority. The official rules are our starting point.

APNAP

Magic is an interactive game. It is not only about executing your strategy, but preventing your opponent(s) from doing the same. As such, there will be times when multiple players want to cast spells or activate abilities at the same time (or in response to someone else doing the same). And, there are times when triggered abilities all "go off" at the same time. APNAP is the way Magic determines the order. The basic rules here:

101.4. If multiple players would make choices and/or take actions at the same time, the active player (the player whose turn it is) makes any choices required, then the next player in turn order (usually the player seated to the active player’s left) makes any choices required, followed by the remaining nonactive players in turn order. Then the actions happen simultaneously. This rule is often referred to as the “Active Player, Nonactive Player (APNAP) order” rule.

101.4d If a choice made by a nonactive player causes the active player, or a different nonactive player earlier in the turn order, to have to make a choice, APNAP order is restarted for all outstanding choices.

Reading the other references to APNAP in the official rules, the basic point is any time things happen simultaneously, the APNAP rule decides in what order. Interestingly,

405.3. If an effect puts two or more objects on the stack at the same time, those controlled by the active player are put on lowest, followed by each other player’s objects in APNAP order (see rule 101.4). If a player controls more than one of these objects, that player chooses their relative order on the stack.

So APNAP may 'force' the active player to put their objects on the stack first [lowest], meaning they will get resolved last.

APNAP applies to all aspects of the game (there is a reason this concept is defined in the "Golden Rules" section of the document). This includes attacks:

802.4. If more than one player is being attacked, controls a planeswalker that’s being attacked, or protects a battle that’s being attacked, each defending player in APNAP order declares blockers as the declare blockers step begins. (See rule 101.4 and rule 509, “Declare Blockers Step.”) The first defending player declares all their blocks, then the second defending player, and so on.

Most of the time, order in such circumstances will not matter. But there will be occasions where it does, so understanding APNAP is important.

Priority

Related to APNAP is the concept of priority. Select rules:

117.1. Unless a spell or ability is instructing a player to take an action, which player can take actions at any given time is determined by a system of priority. The player with priority may cast spells, activate abilities, and take special actions.

117.3. Which player has priority is determined by the following rules: 

117.3a The active player receives priority at the beginning of most steps and phases, after any turnbased 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). 

117.3b The active player receives priority after a spell or ability (other than a mana ability) resolves. 

117.3d 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, they announce what mana is there. Then the next player in turn order receives priority. 

117.4. 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.

Summarizing, the Active Player (he/she whose turn it is) starts with priority and can take actions (cast spells, activate abilities, etc.). When they are done, they pass priority to the Non-Active Player on their left. When he/she takes actions and then passes, priority goes to the next Non-Active Player, and so on, and until everyone passes. Then the objects on the stack resolve. And, per 117.3b, as things resolve on the stack, the active player has a chance to 'react' at each point if they so choose. In thoses cases,

117.7. If a player with priority casts a spell or activates an activated ability while another spell or ability is already on the stack, the new spell or ability has been cast or activated “in response to” the earlier spell or ability. The new spell or ability will resolve first. See rule 608, “Resolving Spells and Abilities.”

Two-player Magic games rarely get into APNAP/priority weirdness, but for multiplayer games, it can get messy. Knowing these rules can help disentangle things.