The structural efficiency of the intersection shell depends on the angle of the intersection of the surfaces. If the angle is small (called here for descriptive purposes, sharp), then a natural rib is formed by the adjacent elements of the basic shells which is much stiffer than the adjacent shells on each side. An itersection for which the angle is very large is called here a shallow intersection. An intersection of 90 degrees is the optimum value because it gives a stiff rib. On large structures with shallow intersections, massive ribs may be necessary which are very evident and detract from the light appearance.