Rules of Service, and Numbering of the Levites Qualified for Service. - After the adoption of the Levites for service at the sanctuary, in the place of the first-born of Israel, Moses and Aaron mustered the three families of the Levites by the command of God for the service to be performed by those who were between the ages of 30 and 50. The particulars of the service are first of all described in detail (vv. 4-33); and then the men in each family are taken, of the specified age for service (vv. 34-49). The three families are not arranged according to the relative ages of their founders, but according to the importance or sacredness of their service. The Kohathites take the lead, because the holiest parts of the tabernacle were to be carried and kept by this family, which included the priests, Aaron and his sons. The service to be performed by each of the three Levitical families is introduced in every case by a command from God to take the sum of the men from 30 years old to 50 (see Num 4:1-3, Num 4:21-23, Num 4:29 and Num 4:30).