These gauges are used to measure slots, grooves, recesses, cylinders, etc., and are especially useful for doing size comparisons. To use, just insert the appropriate gauge into the workpiece and turn the knob at the end of the handle. The spring- loaded ends will "snap" open to the maximum size allowed. Then, tighten the knob, remove the gauge and measure across the tips to find your inside measurement. Each set contains six gauges with a total range of 5/16" to 6" fitted into a handy storage pouch.