The hooked tang on a tape rule is designed to slide in and out a distance equal to its thickness to allow taking inside and outside measurements. Unfortunately, the range of tang movement on many tape rules (even new ones) is less than precise, leading to inaccurate measurements.
Whenever a new tape rule is on my shopping list, I bring a gauge block to the store for checking the tool’s accuracy. The gauge, cut on my tablesaw, has a groove that’s exactly 1" wide, inset exactly 1" from the edge of the block, as shown. Referencing off each side of the groove will allow you to check the accuracy of both inside and outside measurements. (Select a tape with fine increments, and roll the edge of the curve down against the edge of the groove to avoid parallax error.)
The block is also useful for correcting a tang that was bent from accidentally dropping the tool. Just use pliers to bend it back into position, checking the tape rule against the gauge block afterward.
—Paul Anthony, senior editor