Abstract: A corner construction method and apparatus is disclosed including an extrusion construction for mating with and joining a pair of side panels. The extrusion includes a plurality of surface sections defined in cross-section by at least one elongate rib section, squared corners positioned at one end of the rib section, a pair of external leg sections at the other end of the rib section and a pair of internal leg sections parallel to the external leg sections and located along the rib section substantially midway between the squared corners and the external leg sections. A normal projection of each internal leg section onto the adjacent external leg sections ends substantially halfway along the length of the external leg section and apertures are provided for passing screws through the portion of the external leg section beyond that normal projection. The internal leg sections are substantially half agains as thick as the external leg sections.