Numbers 4:29

27At the commandment of Aaron and his sons shall be all the service of the sons of the Gershonites, in all their burden and in all their service; and you shall appoint their duty to them in all their responsibilities.

28This is the service of the families of the sons of the Gershonites in the Tent of Meeting. Their duty shall be under the hand of Ithamar the son of Aaron the priest.

29“As for the sons of Merari, you shall count them by their families, by their fathers’ houses;

30you shall count them from thirty years old and upward even to fifty years old—everyone who enters on the service, to do the work of the Tent of Meeting.

31This is the duty of their burden, according to all their service in the Tent of Meeting: the tabernacle’s boards, its bars, its pillars, its sockets,