The outside micrometer is designed for measuring the outside of objects,also called the outside diameter (OD). The Cclamp and the outside micrometer look similar,which opens and closes by turning an internal screw. In a micrometer,the object that you measure is clamped between the anvil (the stationary end of the clamp) and the spindle (the moving part of the clamp). Once the object is secured in the clamp,you will use the numbering system on the thimble (the handle portion) to find your measurement.