2 Chronicles 8:14

And he appointed, according to the order of David his father,
the courses of the priests to their service
The twenty four courses which served weekly in their turns, ( 1 Chronicles 24:1-31 )

and the Levites to their charges, to praise and minister before the
priests, as duty of every day required;
who also had their courses by lot, to sing the praises of God, when the priests sacrificed, or blew the trumpets, ( 1 Chronicles 25:1-31 ) ,

the porters also by their courses at every gate;
from hence Dr. Lightfoot concludes that these were divided into twenty four classes, as the priests and Levite singers were:

for so had David the man of God commanded,
( 1 Chronicles 26:1-32 ) , who in all these affairs acted as a prophet, under the inspiration and direction of the Holy Spirit of God.