Dasqua precision granite square squareness is +/-.0.0002mm. Squares are made from a solid block of granite that has been carefully machined to ensure accurate right angles Granite precision squares come with one large face and two edges that are finished flat and square to each other. It consists of two faces precisely perpendicular to each other, forming a 90-degree angle Specifically for checking the squareness (90° angle) between 2 surfaces or edges. Check the 90 degree squareness of X & Y axes in its horizontal position. Check the perpendicularity of the Z axes in its vertical position Used with other tools to verify the squareness of various components & assemblies. Particularly useful where high accuracy & reliability in squareness measurements are required This square has a round hole so it weighs less and is easier to handle. An inspection certificate from the manufacturer is provided (not NIST traceable). This square weighs 20 lbs