Jamieson, Fausset and Brown on Joshua 21:4-8

4-8. the lot came out for the families of the Kohathites--The Levites were divided into Kohathites, Gershonites, and Merarites. Among the former the family of Aaron were exclusively appointed to the priesthood, and all the rest were ranked in the common order of Levites. The first lot was drawn by the Kohathites; and the first of theirs again by the priests, to whom thirteen cities were granted, and ten to the rest of the Kohathites (Jos 21:5); thirteen to the Gershonites (Jos 21:6), and twelve to the Merarites (Jos 21:7).