John G wrote:
There is no need to compare squares.

The simple method is to draw a line across a smooth clean piece of
stock, metal or timber, then turn the square over and draw another line
across the stock on top of the first.
If the square is any good the lines will be as one but any error will
show them diverging at twice the error.

Thats good enough for woodworking but not hardly for machinists.
...lew...