The Fowler 52-475-500-0 Hardened Steel Block and Clamp Set comes with two surfaces: X-Blox surfaces that are square and parallel within 0.0005", and V-grooves that are square and parallel with all surfaces within 0.0005". The corresponding dimensions for each pair are alike and within 0.0005". The set of V-block and clamp is hardened for durability and wear resistance and is perfect for holding, grinding, drilling, milling, EDM inspection on surface plates or optical comparators.