A micrometer is either (a) a device used to measure very small distances, usually accurate to 1/1,000 of a millimeter, or (b) a metric measure that is exactly 1/1,000 of a millimeter, or 1/1,000,000 of a meter. It is a device widely used in mechanical engineering for precisely measuring thickness of blocks, outer and inner diameters of shafts and depths of slots. Appearing frequently in metrology, the study of measurement, micrometers have several advantages over other types of measuring instruments like the Vernier caliper - they are easy to use and their readouts are consistent.
The major components and markings of a micrometer include the frame, anvil, spindle , locknut , sleeve , sleeve numbers , sleeve long line , thimble marks , thimble , and ratchet. Micrometers are calibrated in either inch or metric graduations and are available in a range of sizes.