"Pocket" square measures extra large
To check sheet goods for square, I use a collapsible, square-checking triangle. All it takes is the tape from a broken tape measure and some 1⁄16 " pop rivets.
The triangle shown here is cut to 3', 4', and 5' legs, but you could make the sides any length so long as you maintain a ratio that is exactly 3-4-5. Tape the legs together at each corner and drill a 1⁄16 " hole through both pieces at each intersection. Pop-rivet two of the angles together, leaving the joints loose to allow both legs at each rivet location to rotate. For the third angle, insert the rivet into only one leg, leaving the two sides unattached.
When checking a project or a sheet of plywood for square, connect the legs of the open angle by loosely hooking the rivet into the hole. For easy storage, you can fold the triangle into a straight line, and roll it up.
—Mark Thorsell, Golden Valley, Minn.