Straight to better than +/-13.5 microns (0.00053") over the entire length on both edges.
Kinex manufacture straightedges in a wide range of accuracies. We went for DIN 874/2 as it is a good grade for general engineering and fine cabinetmaking without being too expensive.
If you want to measure whether something is or isn't 4 thou, then +/- half a thou represents a 12.5% maximum potential error. Plenty straight enough to tell the difference between 3, 4 or 5 thou.
Stress relieved spring steel
Conforms to DIN874/2