The viola organista is a musical instrument designed by Leonardo da Vinci. It uses a friction belt to vibrate individual strings (similar to how a violin produces sounds), with the strings selected by pressing keys on a keyboard (similar to an organ). Leonardo's design has intrigued instrument makers for more than 400 years, but though similar instruments have been built, no extant instrument constructed directly from Leonardo's incomplete designs is known. Sometimes it is mistakenly referred to as the harpsichord viola, which is a different instrument. Description Leonardo designed many different and elaborate models of viola organista, as preserved in his notebooks of 1488–1489 and in the drawings in the Codex Atlanticus and Manuscript H. The first known instrument actually constructed using Leonardo's concept, is Hans Heyden's Geigenwerk of 1575. While the concept is the same, the design is very different; modern versions of the instrument have been more or less based on Heyden's design. An etching of the Geigenwerk from 1620 shows an instrument of about the stature and shape of a harpsichord (i.e., a piano-shape with flat sides and hard angles). The fully chromatic keyboard appears to have 44 keys, extending about three-and-a-half octaves from low E to a high B. (see illustration) This keyboard range is consistent with harpsichords of the day, although harpsichord keyboards more typically started on low C or low F. The number of strings can't be directly determined, but the design implies one string per key, or 44 strings. In the etching there appear to be five bow-wheels in the instrument, and a foot-pedal, presumably for turning the wheels. A scale given below the drawing has numbers from one to six; if it assumed these refer to feet, then the instrument is a bit over 6 feet long, which again is a scale in keeping with harpsichords of the day. The actual tuning of the instrument is not specified, but given the number of keys and the size of the instrument, it probably extended from around E2 to B5. Variations on this design have been produced over the past four centuries, with varying range, number of keys, and number of bow-wheels. Construction It is not known if Leonardo ever built a working prototype of this instrument. The first similar instrument actually to be constructed was the Geigenwerk of 1575 by Hans Heyden, a German instrument inventor. However, the Geigenwerk is not based on