The blades of Moore & Wright 400 series BS939 Grade B Engineers' squares are straight to a maximum total deviation of 8 microns, the working faces of the stock are ground to a unilateral (concave) tolerance of 4 microns and the two are square to each other (inside and outside) to a maximum total deviation of 16 microns.
Engineers' squares have largely replaced conventional woodworking squares because of their superior ability to remain square.
A square is a tool for transferring squareness. They should be regularly checked against an inspection square (that is never used for anything else) and adjusted if necessary by judicious tapping of the corners against a soft but sturdy surface.
These Moore & Wright 400 series Engineers' squares are very accurately manufactured bits of kit indeed. I took a random selection of 15 of them in various sizes and systematically compared the inside and outside of each one with the outside and inside of every other one - spot on throughout!
- BS939 Grade B