It was an astronomical instrument suitable for the precise measurement of vertical angles in astronomy and geodesy. Its subdivisions were originally in parts of the right angle and later (when the exact division of the circle was possible) in degrees.
Its existence is concluded indirectly by its later incorporation into more complex devices (e.g. the dioptra of Heron). It consisted of a rotating base with a vertical disposition and a rotating calibrated disc with an aiming disposition. It is certain that they also used level goniometers to accurately measure the angles in the horizontal plane.