1 Chronicles — Chapter 24

And David distributed them, that is, Sadoc of the sons of Eleazar, and Achimelech of the sons of Ithamar, according to their courses and ministries. Lapide: David did this not by his own royal authority (a king has no right over sacred things) but through the two high priests Sadoc and Achimelech, acting as channels of the pontifical authority.

And he divided them by lot, both one and the other: for there were princes of the sanctuary and princes of God, both of the sons of Eleazar and of the sons of Ithamar. Lapide: The division by lot ensures that no one can complain of unjust preference, and that each receives their order and turn by God's disposing will. The twenty-four priestly classes (cf. Luke 1:9, Zacharias \"of the course of Abia\") serve in weekly turns throughout the year.