Numbers 3:14

14-31. Number the children of Levi--They were numbered as well as the other tribes; but the enumeration was made on a different principle--for while in the other tribes the number of males was calculated from twenty years and upward [Nu 1:3], in that of Levi they were counted "from a month old and upward." The reason for the distinction is obvious. In the other tribes the survey was made for purposes of war [Nu 1:3], from which the Levites were totally exempt. But the Levites were appointed to a work on which they entered as soon as they were capable of instruction. They are mentioned under the names of Gershon, Kohath, and Merari, sons of Levi, and chiefs or ancestral heads of three subdivisions into which this tribe was distributed. Their duties were to assist in the conveyance of the tabernacle when the people were removing the various encampments, and to form its guard while stationary--the Gershonites being stationed on the west, the Kohathites on the south, and the families of Merari on the north. The Kohathites had the principal place about the tabernacle, and charge of the most precious and sacred things--a distinction with which they were honored, probably, because the Aaronic family belonged to this division of the Levitical tribe. The Gershonites, being the oldest, had the next honorable post assigned them, while the burden of the drudgery was thrown on the division of Merari.

Robert Jamieson, A. R. Fausset & David Brown, 1871 · public domain

The Levites were in three classes, according to the sons of Levi; Gershon, Kohath, and Merari; and these were subdivided into families. The posterity of Moses were not at all honoured or privileged, but stood upon the level with other Levites; thus it was plain, that Moses did not seek the advancement of his own family, or to secure any honours to it. The tribe of Levi was by much the least of all the tribes. God's chosen are but a little flock in comparison with the world.

Matthew Henry, 1706 · public domain