v1-3: Eleazar had become high priest after Aaron. The deaths of Nadab and Abihu are mentioned several times in Scripture as a warning to us.

By the time of David, there were two lines of priests; through Ithamar, Ahitub, Ahimelech, Abiathar (Eli seems to have been in this line also); and through Eleazar, Zadok, Ahimaaz.

Any rivalries were laid aside, as the temple duties were assigned.

v4-6: The duties were chosen by lot, ie: "divided .. impartially". They were written down, to avoid dispute, and to ensure accountability.

v7-18: A list of the 24 divisions. In the N.T., Zacharias, father of John Baptist, was of the line of Abijah (v10), Luke 1.5.

Jewish Rabbis later taught that the 24 courses of priests was established by Moses, albeit at a lower number. Samuel or David were said to have increased this to 24. The logic was that older regulations were deemed to enjoy greater authority. Thus the practical regulations of Chronicles became a means by which later religious authorities sought to control the people.

v19-30: The other Levites, also divided into 24, had other duties.

v31: Younger brethren served with their fathers; as in 25.8, teacher served with student. Here is a training ground, and a system to ensure continued faithfulness (2 Tim 2.2).