Calipers are used to measure diameter and depth or to replicate a measurement from one space to another. Calipers feature a main scale of measurement, a secondary scale of measurement, inner jaws to measure internal diameter, outer jaws to measure external diameter, a locking screw to hold the jaws in place, and, in most cases, a depth probe.
Vernier calipers are the most commonly used. The vernier scale is so precise that it measures the points between the marks on a standard inch or metric scale. For example, if the primary scale has markings for every 10th of an inch, the vernier scale has markings to measure 1/1000th of an inch.
The granularity of vernier calipers makes them difficult to read, but they also come with the option of digital or dial display. Digital calipers have the added benefits of being able to convert from English to metric with the touch of a button. They can also plug into a computer, and because they have no moving parts, they don’t have the toothed rack of the dial calipers.
Though useful for accurate readings, adding a digital or dial display has its drawbacks. In the case of the dial display, a toothed rack is added to the sides of the main scale, which can get clogged with metal chips and throw off the measurement. A display, digital or dial, could also be damaged by fluids, magnetic fields, and shock and so they are not as rugged as standard vernier calipers.
While calipers are great for measuring items with consistent thickness, a micrometer may be necessary if your item has a lip, rim, or bevel on the edge.