Double quadrant, comprising two brass plates and an identically shaped wooden tablet between them. The three plates rotate on one another around a single point located at the apex of the right angle. One quadrant displays on both sides the sine grid, known as the reduction quadrant, the degree scale, the shadow square, and the primum mobile quadrant designed by Petrus Apianus. The other quadrant carries an astrolabe quadrant based on the model developed by Oronce Finé, and a zodiacal quadrant with ecclesiastical calendar. The symbol "II" of the Gemini sign etched on the instrument reveals its maker, Thomas Gemini. Provenance: Robert Dudley bequest to the Medici collections.