A seat assembly including a base and backrest having integrally formed coupler sockets that each receives respective portions of at least one coupler that preferably is an elongate angled coupling tube that joins the base and backrest while serving as a frame. The base and backrest have mating surfaces that abut when assembled to the at least one coupler with one mating surface having a tongue that is received in a groove formed in the other mating surface helping to keep the coupler seated in at least one of the sockets. In a preferred embodiment, each socket includes an elongate channel. In a preferred embodiment, a pair of coupling tubes are used with mounting portions exposed to which seating hardware is attachable. Integrally formed sockets can also be provided for attachment of other features that include armrests and/or hip restraints.