PR: BLACK GRANIT PRISMS Ideal for testing cylinders. The tolerances in the table (in thousandths of a millimetres) refer to the flatness of the base and the parallelism between the base and the axis of the V-shaped groove. On request, other faces can be lapped. CU: CUBES IN BLACK GRANITE Manufactured with either two, four or six lapped, plane and parallel surfaces. See table for dimensions and tolerances. With threaded holes on all faces on request.