John Gill Commentary 1 Chronicles 24:1

"And the courses of the sons of Aaron [were these]. The sons of Aaron: Nadab and Abihu, Eleazar and Ithamar." — 1 Chronicles 24:1 (ASV)

Now these are the divisions of the sons of Aaron Into the classes or courses following:

the sons of Aaron; Nadab and Abihu, Eleazar and Ithamar (1 Chronicles 6:3) , these were the immediate sons of Aaron; but the division or distribution of them into classes are of their posterity in the times of David, who descended from the two latter.