Abstract: A novel system of connectors and elongated tubes joined together thereby to form a variety of rigid three-dimensional rectangular structures. The connectors of the present invention have a central body with a plurality of outwardly extending arms, the arms to seatably engage open ends of the tubes. The connectors are made from extruded components and hence require a minimum of machining. The elongated tubes have notched ends permitting tubes, internally situated within a three-dimensional structure formed therefrom, to be removed or reconfigured.