A caliper (British spelling also calliper) is a device used to measure the distance between two symmetrically opposing sides. A caliper can be as simple as a compass with inward or outward-facing points. The tips of the caliper are adjusted to fit across the points to be measured and after the caliper is removed, the distance is read by using a measuring tool (ruler) between the tips of the caliper.
This version of the caliper allows users to read the value directly from a single display. Many digital calipers can be switched between metric and imperial units. All calipers provide the option for zeroing the display at any point along the slide, allowing the same sort of differential measurements as with the dial caliper but without the need to read numbers that may be upside down. Digital calipers may contain some sort of "reading hold" feature, allowing the reading of dimensions even in awkward locations where the display cannot be seen.