Charles Ellicott Commentary 2 Chronicles 31:17

"and them that were reckoned by genealogy of the priests by their fathers` houses, and the Levites from twenty years old and upward, in their offices by their courses;" — 2 Chronicles 31:17 (ASV)

Both to the genealogy. —The verse is a parenthesis relating to the registration of the priests and Levites, suggested by the occurrence of the word “register” in 2 Chronicles 31:16. Translate, “And as to the register of the priests, it was according to their clans (Hebrew, father-houses); and the Levites from twenty years old and upwards were in their offices in their divisions.” For the latter statement, see 1 Chronicles 23:24.