The guideline describes procedures for the design/type testing, original/receiving inspections as well as the monitoring inspection of steel squares 90°. The necessary work procedures for the respective examinations are described in form of directly applicable work instructions. Apart from the calibration also the preparatory work for the calibration unit as well as for the inspection items are described. Further help is given for the evaluation and the documentation of the calibration. The guideline is valid in connection with Part 1.1 and Part 1.2.