Kinex 4001-03-100 Machinist Knife Edge Solid Inspection Square 4&rdquo; x 2-3/4&rdquo; x .240&rdquo; Thick (100 mm x 70 mm) DIN 875/00 (Square w/in 3 microns or 0.0001&rdquo;) Overall size: 4" x 2-3/4" x 0.240" Adheres to strict German DIN 875/00 accuracy standards- squareness guaranteed to w/in 3 microns or 0.0001 (that's one ten thousandth of an inch!) Machined from a solid blank of tool steel (will never go out of square) Wide enough to stand on its edge on its own Made by Kinex Measuring