Zadok and Ahimelech (rather Abiathar, see 1 Chronicles 24:6) assisted David in drawing up the priestly courses, as the "captains of the host" assisted him in making the divisions of the singers 1 Chronicles 25:1.

And Ahimelech - Ahimelech is put here for Abiathar, who was high priest in the days of David. Abiathar had also the name of Ahimelech, as well as his father. See Calmet.

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.

And David distributed them, both Zadok of the sons of Eleazar, and Ahimelech of the sons of Ithamar, according to their offices in their service.

24:3 Distributed - Allotting to each of them several times, wherein they should by turns have the government of holy ministrations.

3. Zadok . and Ahimelech of the sons of Ithamar-This statement, taken in connection with 1Ch 24:6, is not a little perplexing, since (2Sa 15:24, 35; 20:25) Abiathar is mentioned as the person conjoined in David's time with Zadok, in the collegiate exercise of the high priesthood. Some think that the words have been transposed, reading Abiathar, the son of Ahimelech. But there is no ground for regarding the text as faulty. The high priests of the line of Ithamar were the following: Ahiah or Ahimelech, his son Abiathar, his son Ahimelech. We frequently find the grandfather and grandson called by the same name (see list of high priests of the line of Eleazar, 1Ch 6:30-41). Hence the author of the Chronicles was acquainted with Ahimelech, son of Abiathar, who, for some reason, discharged the duties of high priest in David's reign, and during the lifetime of his father (for Abiathar was living in the time of Solomon, 1Ki 2:27) [Keil].

24:1-31 The divisions of the priests and Levites. - When every one has, knows, and keeps his place and work, the more there are the better. In the mystical body of Christ, every member has its use, for the good of the whole. Christ is High Priest over the house of God, to whom all believers, being made priests, are to be in subjection. In Christ, no difference is made between bond and free, elder and younger. The younger brethren, if faithful and sincere, shall be no less acceptable to Christ than the fathers. May we all be children of the Lord, fitted to sing his praises for ever in his temple above.