Both of these two calipers cannot give you a measurement unless you use them with a steel rule. They can be used as a go/no-go gauge to determine if something is the same diameter as something else. To determine a millimeter measurement you have to place them on a steel rule and read the distance between the legs in millimeters. Different forms of these calipers exist as well. The images above show the external version, whereas the images below show the internal version. 

Its body is made from hardened stainless steel and its jaws can be used to scribe lines onto metal or sheet metal. It is mostly used to measure the thickness of a material, the depth of a cut or drilled hole, and the diameter of a round object or hole. Because it is such a highly accurate measuring tool care should be taken when handling and using it. Do not throw it or use it as a pickaxe. Do not drop it. Keep it clean and lubricated. Reading a vernier is easy, and will covered in US 4433.

This tool is able to measure the thickness of a human hair. Because it is so precise and accurate, it is also delicate and can be easily damaged or knocked out of calibration. Always use a micrometer with care and do not over-tighten the adjustable jaw. Only tighten it on the ratchet knob on the end of the handle to prevent over-tightening it. Store it in its box when not in use. As with the vernier, learning how to read it will be covered in US 4433.