Checking a square for square. (Video)

[Mike Baker 2]

https://youtu.be/qSU8nVtVGKQ

Ignore the preview; Youtube is blowing a gasket and can't seem to use the one I provided. Don't know what that's supposed to be.

[Roger Nair]

This video is on the right track but with care a tighter tolerance can be achieved. A factory edge on wood products is not a guaranteed straight edge. Try to true the edge and compare with a known straight edged tool. The blade of the square should evaluated for wear, parallel and distortion. When striking lines, use a sharp pin point awl instead of a fat wearing in use pencil. Strike close parallel lines, the deviation from parallel is twice the error of the square.