Abstract: The gauge is a flat metal block with a precisely finished planar bottom face by which it can be supported on the upper planar face of a conventional granite gauge block for lateral sliding therealong while retaining a fixed angular relation to the planar face of the block. The base has an open end V-slot with planar faces which extend at 90.degree. to the plane of the bottom face, from the bottom to the top of the base, and which diverge outwardly at 90.degree. to each other to, and open through, a side of the base, to provide positioning jaws. The base, while resting on the granite block, can be slid to cause these faces of the positioning jaws to engage the circumferential wall of an article of circular cross section while one end of the article rests on the face of the granite block.