This surveyor's square consists of a hollow cylinder with eight vertical slits placed around its circumference at 0°, 30°, 90°, 135°, 180°, 210°, 270°, and 315°. The cylinder base has a central socket to fix the instrument onto a tripod (now missing). The upper part is closed by a disk with a graduated rim and a magnetic compass at its center. The compass has a cover and an alidade with viewers. The upper part was used for surveying, while the cylinder with the eight slits was used to establish alignments at right angles and at 30° and 45° angles. The instrument is probably related to the rule inv. 638, 3703, also made in Urbino. Provenance: Medici collections.