How can you tell if your square is square?
Andy Rae: To find out if your square is square, place the square’s fence against the straight edge of a piece of wood and mark a line along the blade’s length. Now flip the square over and slide the blade up to your mark. Any discrepancy from the line is twice the amount of error in the square.
Carol Reed: Place the base of your square against a known straight edge. Draw a line along the blade of the square. Flip the square over so the base of the square is on the opposite side of the drawn line. Draw another line on top of the first line. If it is square, you will not see two lines.
Richard Jones: Find a straight-edged plank. Hold the stock against the straight edge. Scribe a line along the blade. Flip the stock of the square over on the straight edge. The edge of the blade should coincide with your original line squared across. If it doesn’t, your square is nothing more than an interesting piece of metal and wood.