Answer:
Vernier calipers are used, especially in metal engineering, to make very precise measurements of internal and external dimensions of small objects. The calipers slide together, and a reading is taken by visual examination of the sliding scales on the calipers. There are two scales, and careful scrutiny of the second scale enables greater precision than just using one scale.
Although Vernier calipers have been used for many years, and have proven invaluable in ascertaining, within acceptable tolerances of accuracy, i.e. measurements down to fractions of a millimeter, the advert of digital technology makes it possible to measure dimensions to an even greater precision! The down-side of digital calipers, however, is that they need a battery, unless of course, you can get a solar powered one!
For more information about Vernier and digital calipers, how they work, and photographs, see Related links below.