The instrument is composed of two overlapping square plates. The top plate has a large round opening whose rim is engraved with a graduated scale. Inside the opening, pressed against the top plate, is a rotating disk with a central vertical gnomon, a line grid for Italian hours (24 equal hours from sundown), and markings for the cardinal points: T-ramontana [North], L-evante [East], O-stro [South], and P-onente [West].

The declination quadrant was used to find the orientation of a vertical surface such as a wall. The side of the instrument marked "Parte del declinatorio verso il muro" was placed against the wall. The central disk would then be turned until the gnomon's shadow showed the time at which the measurement was performed. The Tramontana-Ostro axis would thus be parallel to the north-south axis, and the angle between the wall and the meridian line could be read on the graduated scale.