Master Precision Square is the finest precision-checking non-graduated square with accuracy to .0001" (0.0025mm) every 6" (150mm). This hardened steel square is used when extreme accuracy is required. FEATURES: The beams and blades are hardened, ground and lapped to ensure parallelism and straightness. Made of high quality tool steel, with the finest of craftsmanship throughout. The beam is grooved at the inner corner for clearance of burr or dirt.