The main phase is both the second and fourth phases of a turn. Non-instants can usually only be played during this phase, only by the active player, and only when the stack is empty.
The following events occur during the main phase:
- Abilities that trigger at the beginning of the main phase go onto the stack.
- The active player gains priority.
Once per turn, the active player may play a land from his or her hand during this phase while the stack is empty. This is considered a Special Action which does not use the stack.
When both players yield priority in succession while the stack is empty during the pre-combat main phase, the game proceeds to the combat phase. After the combat phase is complete, the game proceeds to the post-combat main phase.
When both players yield priority in succession while the stack is empty during the post-combat main phase, the game proceeds to the end phase.
From the Comprehensive Rules (Aether Revolt (January 20, 2017))
- 505. Main Phase
- 505.1. There are two main phases in a turn. In each turn, the first main phase (also known as the precombat main phase) and the second main phase (also known as the postcombat main phase) are separated by the combat phase (see rule 506, “Combat Phase”). The precombat and postcombat main phases are individually and collectively known as the main phase.
- 505.1a Only the first main phase of the turn is a precombat main phase. All other main phases are postcombat main phases. This includes the second main phase of a turn in which the combat phase has been skipped. It is also true of a turn in which an effect has caused an additional combat phase and an additional main phase to be created.
- 505.2. The main phase has no steps, so a main phase ends when all players pass in succession while the stack is empty. (See rule 500.2.)
- 505.3. First, but only if the players are playing an Archenemy game (see rule 904), the active player is the archenemy, and it’s the active player’s precombat main phase, the active player sets the top card of his or her scheme deck in motion (see rule 701.23). This turn-based action doesn’t use the stack.
- 505.4. Second, the active player gets priority. (See rule 116, “Timing and Priority.”)
- 505.4a The main phase is the only phase in which a player can normally cast artifact, creature, enchantment, planeswalker, and sorcery spells. The active player may cast these spells.
- 505.4b During either main phase, the active player may play one land card from his or her hand if the stack is empty, if the player has priority, and if he or she hasn’t played a land this turn (unless an effect states the player may play additional lands). This action doesn’t use the stack. Neither the land nor the action of playing the land is a spell or ability, so it can’t be countered, and players can’t respond to it with instants or activated abilities. (See rule 305, “Lands.”)