An assembly comprises an elongate member and a receiving member. The elongate member has an external cross-sectional shape which is substantially square and has corners replaced by circumferentially spaced circular portions of a circumscribing circle with flat portions therebetween which lie within the circle. The receiving member has a receiving formation having an internal cross-sectional shape which cooperates at least with the flat portions of the external cross-sectional shape of the elongate member to prevent relative movement between the receiving member and elongate member about a longitudinal axis of the elongate member. The assembly is adapted for use in a joining the members of a wheelchair. The invention is further directed towards a structural assembly comprising an elongate member engaging in a receiving formation of a receiving member and a clamp embracing the receiving member to clamp the elongate member in the receiving formation. The clamp includes a locating element cooperating with respective formations in the elongate member and receiving member to position the elongate member in the direction of its length relative to the receiving member. The clamp is adapted for use in a joining the members of a wheelchair.