Abstract: A wall module has a first connecting portion with a non-symmetrical tongue with a tip at one end, an abutment near the opposite end, and a convex arcuate surface between them. The module also has a second connecting portion with a groove having a shape complementary to the tongue, including a blind first end, open second end with an abutment adjacent to it, and an arcuate surface between the ends. An intermediate portion connects the first and second connecting portions, and may be flat or define an open rectangle. Two modules are connected together by inserting the tongue of one into the groove of the other and rotating (e.g. about 90.degree.) until the abutments abut. The modules may be joined by bolts to other modules or an elevator cage.