An Out Of Square Square Just Does Not Work

My most favorite square is the combination square. It's easy to carry around, gives you both 45º and 90º angles as well as being a marking gauge for drawing lines down lumber.

But is it perfectly square? Many are not and over time the landings the blade rests on wear. So from time to time these squares need a little bit of calibration.

Here's the easy way to do just that: