The flip test
- Register the stock against a known straight edge — the factory edge of a sheet of MDF will do.
- Mark a line along the blade.
- Flip the square over, register from the same point, and mark again.
Two lines that diverge tell you the error, doubled. If they overlap, the square is good.
What to do with a failure
Keep it for marking parallel lines and rough work, and label it so it never gets used to check an assembly. A square that is out by a fraction of a degree will put that error into every joint it touches, consistently, which is worse than an obviously broken tool.
Why a knife beats a pencil
A pencil line is half a millimeter wide and you have to decide which side to cut on. A knife line has no width, and it severs the surface fibers so a chisel drops into it — which is how a crisp shoulder happens.
What comes up most often
What tolerance should I expect?
Good engineer squares are specified in hundredths of a millimeter over their length. Most woodworking squares state nothing at all, which is why the flip test matters more than the label.
Last reviewed 2026-08-12