A connection surface that is disposed on at least a portion of a face of a structural unit comprises a first convex surface and a first concave surface arranged substantially along a first line. The shape of the first concave surface is complementary to the shape of the first convex surface. A second convex surface and a second concave surface are arranged substantially along a second line that is substantially parallel to the first line. The shape of the second concave surface is complementary to the shape of the second convex surface. The concave-convex order of the second line is reversed relative to the first line, such that the second convex surface corresponds with the first concave surface and the second concave surface corresponds with the first convex surface.