Calipers are used for measuring and checking small linear dimensions. They are usually made of metal or fibreglass-reinforced plastic. Using this manually operated measuring instrument, internal dimensions, external dimensions and depth dimensions of objects such as screws or nuts can be determined. Calipers consist of a rail which has a sliding adjustable piece. In addition to calipers with engraved numbers, there are also dial calipers and calipers with a digital display. Calipers are used for measuring the dimensions of mechanical components, for example.