illustration of how to tell if your your combination square is square as part of a this old house tool test
Illustration: Gregory Nemec
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Check It for True

With a tool prized for its accuracy, it's worth checking every now and then to make sure yours is true.

Extend the blade as far as possible, grooved side up, through the 90-degree face of the head, and hook the square against the straight edge of a board. Scribe a line along the blade. Now flip the tool over, groove down, and line up the same edge of the blade with your mark. If they’re parallel, the tool is square. If not, it needs to be repaired or replaced.
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