The Starrett 20 Master Precision Square is the finest precision-checking square – not graduated. Squareness accuracy to .0001" (0.0025mm) every 6" (150mm). This hardened steel square is used when extreme accuracy is required. The beams and blades are hardened, ground and lapped to ensure parallelism and straightness. The beam is grooved at the inner corner for clearance of burr or dirt. Made of high quality tool steel, with the finest of craftsmanship throughout. 3" (75mm) Blade, 2 3/8" (60mm) Beam throughout.