This geometrical square consists of a frame inscribed with the shadow square on two contiguous sides. At the corner opposite the one formed by the shadow square is a hinged shutter with sights. Inside the frame are a quarter-circle with the degree scale and a diagonal rod carrying a compass and magnetic needle at its center. A pillar attached to the back of the square allows the instrument to be installed on a wooden base and kept in a horizontal position during surveying work. This specimen resembles the model published by Georg von Peurbach in his treatise Quadratum geometricum (Nuremberg, 1516). Provenance: Medici collections.