DIN 877 classes 1a and 1b. 1 = DIN 877 class 1a with very high sensitivity longitudinal vial 0.02 mm/m. 2 = DIN 877 class 1b with high sensitivity longitudinal vial 0.1 mm/m. Sturdy frame of high quality special casting hammer finish. 3 sides Vee ground, 1 side flat, each with one longitudinal and one transverse vial of acrylic glass. The sensitivity (scale value) of the longitudinal vial is specified on the square. Accuracy grade 1 has insulated handles on the housing and protective glass cover over the vials. For aligning horizontal and vertical faces, shafts etc. and for accurate alignment at a 90° angle.