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.
505.1b
Phrases such as “first main phase,” “second main phase,” and so on count the number of main phases that have occurred only in the current turn unless that text specifies otherwise.