Four-point proportional compasses whose legs are fitted with steel tips and slots for sliding the clamp and pivot. The pivot divides each leg into two parts, whose length ratio determines the operations indicated on the proportional scales engraved on the slits. There is a polygon scale on the front. The back is engraved with scales for squaring the circle and dividing the circumference. The sides of each leg carry scales for line division and for regular bodies. The instrument is signed by Giovanni Maccari, who also made the Galilean sector inv. 3540.