Compass, divider, and calliper are basically instruments that have two legs pivoted to each other at the top and are concerned with small-distance measurement or transfer. The compass and divider have straight legs; the calliper has curved legs.
The terms compass and divider are often interchanged, for each instrument can be used to draw circles, mark divisions (divide a given distance), or simply mark a distance. Technically, a compass is a drafting instrument that has one pen or pencil point and one sharp point that is positioned at the centre of the circle to be described, while a divider, on the other hand, has two sharp points, one for the centre and the other for scribing or marking. Calliper is a corruption of calibre, the diameter of a hole (as in a firearm) or of a cylindrical or spherical body. The outside calliper has inwardly curved legs that measure the diameters of solids created by rotating tools, such as lathe-turned objects, and the inside calliper has outwardly curved legs for measuring bores.
Dividers and callipers were known to both the Greeks and Romans, though the calliper was uncommon. A divider with a circular sector, or wing, connecting the two legs was sketched in 1245; its modern counterpart is the wing divider with a thumbscrew clamp and screw for fine adjustment. The calliper is mentioned in the Middle Ages, but the divider was the principal tool of the architect working on full-scale layouts of stonework, such as in the construction of a cathedral. Such dividers were large, often half as tall as a man. The divider underwent refinements that made it an important drafting instrument for Albrecht Dürer and Leonardo da Vinci; Leonardo suggested improvements that included the knuckle-joint hinge (to increase rigidity) and the adjustable proportional divider (Roman proportional dividers had a fixed pivot that gave only one ratio). Leonardo’s notes also show the beam compass with a screw adjustment for large radii, as well as a compass that had interchangeable points, in which one leg had a clamp for different drawing media, such as graphite or chalk.