1 Chronicles 24:3

3 With the help of Zadok, who was a descendant of Eleazar, and of Ahimelech, who was a descendant of Ithamar, David divided Aaron’s descendants into groups according to their various duties.

1 Chronicles 24:3 Meaning and Commentary

1 Chronicles 24:3

And David distributed them
The posterities of Eleazar and Ithamar into certain classes or courses:

both Zadok of the sons of Eleazar, and Ahimelech of the sons of
Ithamar;
these were the two principal men of the respective families, the one was high priest, and the other "sagan" or deputy; or they both officiated as high priests alternately, until the one was removed in Solomon's time, and the other established alone. And now their brethren the priests, who were under each of these, are the persons David divided into classes:

according to their offices in their service:
to perform their office as priests in the service of the temple in their turns.

1 Chronicles 24:3 In-Context

1 This is how Aaron’s descendants, the priests, were divided into groups for service. The sons of Aaron were Nadab, Abihu, Eleazar, and Ithamar.
2 But Nadab and Abihu died before their father, and they had no sons. So only Eleazar and Ithamar were left to carry on as priests.
3 With the help of Zadok, who was a descendant of Eleazar, and of Ahimelech, who was a descendant of Ithamar, David divided Aaron’s descendants into groups according to their various duties.
4 Eleazar’s descendants were divided into sixteen groups and Ithamar’s into eight, for there were more family leaders among the descendants of Eleazar.
5 All tasks were assigned to the various groups by means of sacred lots so that no preference would be shown, for there were many qualified officials serving God in the sanctuary from among the descendants of both Eleazar and Ithamar.