The dimensional reading on this caliper used by Robert Marichy, a wood turner, is carried out on a ‘vernier’. Invented in 1631 by the Franche-Comté mathematician Pierre Vernier, the eponymous vernier is a graduated ruler affixed to the calipers and certain goniometers intended for measuring angles. Thanks to the direct coincidence of two lines, it makes it possible to quickly and easily read the measured values. Today some calipers are equipped with liquid crystal or digital displays.