"Bill Rittner" wrote in
news:euCdb.2778$N94.2577@lakeread02:

To check a square hold it to the edge of a straight board and scribe a
line, then flip the square over and, using the same edge, check the
line you scribed. The tool is out of square by 1/2 the amount of error
you see.

Of course, that's only as accurate as the straightness of the edge
of the "straight" board you're using. You may be measuring the
curve of the edge of the board (times 2, of course). Which then
begs the question "how do I test an edge for straightness" :-)

John