A vernier caliper is a length measuring instrument composed of a fixed graduated ruler, provided with a flat-faced head corresponding to the zero reference position, and a movable cursor, provided with a flat-surfaced head in opposition to the reference face. Its vernier measurement system is based on the juxtaposition of two graduations (in millimeters and in fractions of millimeters). This instrument makes it possible to precisely and easily measure the external dimensions of an object - like here a wooden wheel - the diameter of a cylinder or even a depth.