A girder that is curved in plan view—a circular ring girder—only needs to have a hinged support along a single line to keep it from flipping downward. If it has bearings on the inside edge, the line load causes ring tension on the upper side and ring compression on the lower side of the slab, whereas it is the other way round for a line support on the outer edge. The efficient form of a curved arch in plan view is manifested in the principle of inversion: The arch is shaped the same as a hanging cable.