I've been using one of these for years. thousand times. It is an essential tool in my main tool bag. always with me, in every project. any type of project. For those who don't know what a vernier caliper is, this tool accepts 3 types of measurements: OD, ID, and depth. The main goal is the outside diameter, which represents the main jaws. Just roll your thumb, unroll the reel and read the dial. The smaller cheeks on the back are "inverted". You use them to measure the inside of a hole orβ¦