The rule that holds the button when a mix moves from Hold'em to Stud games.
TDA Rule 12-B. After the last Hold'em-type hand, the button moves to where it would go if the next game were also Hold'em-type, and stops there. It doesn't move during Stud (the cards decide the order of action). When returning from Stud, play resumes from that position, and the 2 players to its left post the blinds.