When David was not permitted to build the temple, he proceeded, among the last acts of his life, with the assistance of Zadok and Ahimelech, to organize the priestly and musical services to be conducted in the house of God.
1) He divided the priests into twenty-four courses (1 Chr. 24:1-19), sixteen being of the house of Eleazar and eight of that of Ithamar. Each course was under a head or chief, and ministered for a week, the order being determined by lot.
2) The rest of the 38,000 Levites (23:4) were divided also into twenty-four courses, each to render some allotted service in public worship: 4,000 in twenty-four courses were set apart as singers and musicians under separate leaders (25); 4,000 as porters or keepers of the doors and gates of the sanctuary (26:1-19); and 6,000 as officers and judges to see to the administration of the law in all civil and ecclesiastical matters (20-32).
This arrangement was re-established by Hezekiah (2 Chr. 31:2); and afterwards the four sacerdotal courses which are said to have returned from the Captivity were re-divided into the original number of twenty-four by Ezra (6:18).