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To determine whether a carpenter’s square is true, all you need is a straight line. Sound too simple?
First you lay the square to be tested flat, its blade tight to the line. Then draw a second line along the tongue.
Now flip the blade over, and snug the heel to the square into the (supposedly) ninety-degree angle next to the one you’ve just drawn. Draw a third line.
It the square is true, lines two and three should be the same and are, therefore, perpendicular to the first: Your square is indeed square.
If not…Well, that tool is no fair square.