Thes were the divisions of the sons of Aaron. The sons of Aaron: Nadab and Abihu, Eleazar and Ithamar.
"And the courses of the sons of Aaron were these.  The sons of Aaron: Nadab and Abihu, Eleazar and Ithamar.  But Nadab and Abihu died before their father, and had no children: therefore Eleazar and Ithamar executed the priest's office.  And David with Zadok of the sons of Eleazar, and Ahimelech of the sons of Ithamar, divided them according to their ordering in the service.  And there were more chief men found of the sons of Eleazar than of the sons of Ithamar; and thus were they divided: of the sons of Eleazar there were sixteen, heads of fathers' houses, and of the sons of Ithamar, according to their fathers' houses, eight.  Thus were they divided by lot, one sort with another; for there were princes of the sanctuary, and princes of God, both of the sons of Eleazar, and of the sons of Ithamar. And Shemlah the son of Nethanel the scribe, who was of the Levites, wrote them in the presence of the king, and the princes, and Zadok the priest, and Ahimelech the son of Abiathar, and the heads of the fathers' houses of the priests and of the Levites; one fathers' house being taken for Eleazar, and one taken for Ithamar."