Pivoted from one corner of the plate is a radial arm with a point. This traverses a quadrant divided into degrees and numbered from 0? to 90? in each direction. The lower end of the radial arm contains a compass, which has remained intact (diameter 23 mm).
The sides of the plate are inscribed with the names of the directions of the compass. To the left of the pivot point for the arm is the inscription: 'LEVANTE A MERIDIANO', to the right: 'PONENTE A MERIDIANO'. The other sides are inscribed: 'SEPT A LEVANTE' and 'SEPT A PONENTE'. The underside of the plate bears no markings.
This quadrant is a declinatory used by makers of sundials when designing a vertical declining sundial. The edge of the plate is held horizontal and placed against the wall. The arm is then moved until the compass needle shows that the arm is aligned with the meridian, when the pointer givers the angle measured from the scale. The correct edge of the plate is chosen for a wall declining East or West. The instrument may also be used for inclining dials. Then the plate is held vertical, and the pivoted arc, weighted by the compass box, hangs in the vertical plane and allows the inclination to be read from the scale.
See G. L'E. Turner, "The Florentine workshop of Giovan Battista Giusti", Nuncius: Annali di storia della scienza, 10 (1995), fasc. 1, pp. 154-5.