Abstract: A modular building system wherein two profiled tubular members are connected to each other by two first cores which are slidably received in one of two elongated channels of one of the members, by two second cores which are received in the two channels at one end of the other member, by two non-circular posts which extend into sockets of first cores, through one wall of the one member and into sockets of the second cores when the one end of the other member is placed against the outer side of the one wall of the one member. First pins are used to secure the first cores to the one member and to the posts in their sockets, second pins are used to secure the second cores to the respective posts, and threaded fasteners are used to secure the second cores to the other member and/or to the posts.