5.  Turn Structure

507.  Beginning of Combat Step

507.1  First, if the game being played is a multiplayer game in which the active player’s opponents don’t all automatically become defending players, the active player chooses one of his or her opponents. That player becomes the defending player. This turn-based action doesn’t use the stack. (See rule 506.2.)

507.2  Second, the active player gets priority. (See rule 116, "Timing and Priority.")