Reply by John

I got a small (~6”) steel “engineers” square from Woodcraft a couple of years ago for about $15. It’s made like a try, with one thick leg and one thin. Handy and very accurate, and I’ve seen them lots of other places. For day-to-day use, I have a cheap Stanley combo square that’s quite square, though definitely not as nice to use as a Starret. Big orange box, $15. Best user is a small (4”) try square made of wood (fat leg) and brass (thin leg). Either Popular Woodworking or Fine Woodworking did an article on making one yourself within the last year or so.

You can check any square for square on a board or other surface with one straight edge. Usually the factory edge on a sheet of plywood is straight if you’re in a pinch. Just use the square in question to draw a line perpendicular to the straight reference edge, flip the square over and draw another. Any deviation from 90 degrees shows up quickly.