It is a surveying instrument mounted on a staff attached to the bridge winglets, graduated in 180° and sometimes from 180° to 360° in another line on the limb. The grophometer was invented in Paris by Philippe Danfrie around the end of the 16th century. It is a variation of the simple theodolite or early azimuth instrument. It is described as a "lensatic compass, suitable for approximate angle measurements. It is a graduated semicircle with two alidades (or limb with sights), one of each is mounted on the semicircle, the other is free to rotate on the graduated limb. The movable alidade is provided with a vernier allowing to read the first ten minutes. Thanks to the large compass in the middle (se the pictures above), the instrument can also be used as a topographic surveying instrument, unlike the older versions designed by Danfrie.