WordNet
n. a small movable scale that slides along a main scale; the small scale is calibrated to indicate fractional divisions of the main scale [syn: vernier]
Wikipedia
A vernier scale is a device that lets the user measure more precisely than could be done unaided when reading a uniformly divided straight or circular measurement scale. It is a scale that indicates where the measurement lies in between two of the marks on the main scale. Verniers are common on sextants used in navigation, scientific instruments used to conduct experiments, machinists' measuring tools (all sorts, but especially calipers and micrometers) used to work materials to fine tolerances, and on theodolites used in surveying.
Usage examples of "vernier scale".
The location grid was calibrated in standard units for easting and northing, with the superimposed vernier scale throwing everything out of focus except the correct reading.