What are calipers?

Calipers are tools which are used to measure things. They consist of two legs attached to a central hinge; the legs can be moved closer together or further apart to obtain a measurement. Different calipers are designed for specific purposes, like measuring the insides of objects and the outsides of objects. Many trades require the use of calipers, and these tools are typically readily available from stores which cater to these trades.
A very basic set of calipers looks rather like a set of pincers. When the pincers curve inwards, as though they are going to grip something, the calipers are used to measure the outsides of objects, like screws, nuts, wooden planks, and so forth. When the pincers curve outwards, the calipers are designed to measure the insides of things, like tubes. Typically the central hinge has a set of markings which can be read to obtain distance measurements.
It is also possible to find calipers mounted to a slide rule, in which case the calipers are moved along the ruler to obtain the desired measurement. One famous caliper of this design is the Vernier caliper, which has two sets of guides which allow the user to measure interiors and exteriors. Vernier calipers often come with multiple systems of measurement in their markings, allowing people to determine distances in both Imperial and Standard measurements, for example. It is also possible to find Vernier calipers with digital readouts which eliminate potential operator error.
In some cases, calipers may be fixed open at a specific width. These fixed calipers are used in quality assessment and to maintain standards; for example, a company might use fixed calipers to ensure that it is producing bolts which are the same size. By contrast, movable calipers are totally adjustable, allowing people to measure a range of objects, although they can sometimes be locked into place for the purpose of standardized measurements.
Calipers are extremely useful measuring tools, and they are very easy to use. Some people find calipers rather addictive when they are first introduced to them, as these devices can be used to measure all sorts of things. In specific trades where calipers are used, experienced people often prefer one type or another. If you are seeking out calipers to assist you with work, you may want to ask other people in your field about the calipers they use to see if they have any recommendations.