No-measure method to check square
If you rely on a tape measure to compare the diagonal lengths of a box or drawer when checking it for square, the wide tip of the tape may make you wonder about its accuracy. So, to take the imprecision (and numbers) out of the equation, build this diagonal gauge. Simply fit hardwood slides into both ends of a length of T-track, and then thread in knobs to secure the slides. Move the two slides until they touch the inside corners, then tighten the knobs to transfer inside measurements.
—Gary Dean, Prince George, B.C.