Stepped groove construction. Made of case hardened steel in numbered pairs. Clamps within grooves permit use on sides. Set consists of two blocks and two clamps. Work capacity to approximately 2" (51 mm) diameter. Accuracy: V-grooves are central, parallel and square with all surfaces within 0.0003". V-groove is 90 degree +0.0000" -0.0003". Corresponding dimensions for each pair are alike within 0.0003". Clamping Range 2". US Made.