A micrometer consists of two opposing surfaces, a stationary anvil and a moveable spindle. On most micrometers, these hardened steel or carbide-tipped contact surfaces are flat. However, micrometers can also be equipped with contact tips with unique forms for measuring special part characteristics.
Convenience is one of the reasons the micrometer is often the tool of choice for length/diameter measurements. The basic micrometer provides direct size information quickly, has high resolution and is easily adaptable to many different measurement applications.
Source: How to choose a micrometer that fits your needs