How do I check an angle with the reversal measurement?

The reversal measurement, often referred to as the reversal method, is a very simple and accurate method of checking the accuracy of a 90° angle. The reason why the reversal measurement is so accurate is that the accuracy of the measurement does not depend on a second test angle. A plate with a straight edge, a pencil and a calliper are all that is needed for this simple method. This method can be used to check all angles, e.g. carpenter's squares, joiner's squares, locksmith's squares, bricklayers' squares and many more.

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