This page has a video demonstrating the Machinist Calipers.
Calipers are extremely versatile measuring tools. They are capable of 4 different types or measurements. You can use them to measure O.D, I.D, Depth and they can also be used as a height gage.
I don’t recommend the caliper as a precision means of measuring. But you can’t beat them as an all around measuring tool, if you are working in tolerances of .002 or so.