1. Here are the groups of priests the sons of Aaron were separated into. The sons of Aaron were Nadab, Abihu, Eleazar and Ithamar.
2. But Nadab and Abihu died before their father did. They didn't have any sons. So Eleazar and Ithamar served as the priests.
3. With the help of Zadok and Ahimelech, David separated the priests into groups. Each group served in its appointed order and time. Zadok belonged to the family line of Eleazar. Ahimelech belonged to the family line of Ithamar.