the two convex segments 38 of the sphere face in opposite directions and are relatively closely spaced to each other to form a relatively thin and flat head 16 by making the radius approximately the length of the illustrated modules as illustrated in the drawings the appropriate seven degree 7 u00b0 or less included angle for frictional locking will be obtained in the present case if the overall length of the module is about three inches the radius 39 could be approximately three inches and centered at 40 as seen in fig9 to form one of the convex spherical frictional locking surfaces the opposed convex spherical locking surfaces 38 forming the head 16 can be visualized by considering diametrically opposed equal segments of the sphere brought close together as seen in fig9 for each of the modules the sockets 20 in the skirts 18 of each of the modulesl 2 are provided with a pair of concave spherical locking surfaces 46 which face each other and are complementary to the spherical convex locking surfaces 38 on the head 16 of an adjacent module the concave locking surface 46 in socket 20 are generated with a radius 45 substantially equal to radius 42 used to form the complementary spherical locking surface 38 with the convex shape referring to fig9 and to the lower one of.