A rigid structural member for a building in which the member has four sides each provided with a groove having narrow opening. Two of the grooves on two opposite sides be used to assemble structural members together to provide completed building assemblies by the use of screw-threaded members while, in the completed assembly, the other two grooves are used for holding two building enclosure sheets in spaced positions, one on each side of the completed assembly. For this purpose, elongate holding members are located in the grooves to hold the sheets in place. The holding members have a particular design which secures them in place within the grooves. The grooves are preferably T-shaped and are all of the same size and shape to increase the versatility. In addition, the structural member may be formed into building structures which are hinged together such that they may be in a collapsed condition for storage and transportation or may be relatively pivoted into an erected condition.