Matthew Poole and continuators on 1 Chronicles 24:31

31 These likewise cast lots over against their brethren the sons of Aaron , in the presence of David the king, and Zadok and Ahimelech, and the chief of the fathers of the priests and Levites, even the principal fathers over against their younger brethren .

i. e. Answerable for Number and Order to those of the Priests, so as there should be a several course of the Levites for each course of the Priests. This is expressed concerning the Singers, ch. 25. and the like is implied concerning the Porters, ch. 26. and is here sufficiently intimated concerning those Levites which were employed in other Sacred Ministrations.

The Lots of the Elder and Younger Brethren were promiscuously put together, and the order was settled as the Lots came forth without any regard to the Age or Dignity or Number of the Persons or Families, the youngest Family having the first Course if they had the first Lot, &c.