Wikipedia
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The Viola pomposa (rarely confused or referred as the Violino pomposo) is a five stringed instrument developed around 1725. There are no exact dimensions applicable to all instruments used under this name, although in general the pomposa is slightly wider than a standard viola (that's why it is called pomposa). It uses four viola strings, tuned conventionally (C-G-D-A), with the addition of a high E string (usually a violin string), giving it a greater range than the orchestral viola; the trade-off comes in a sound which is slightly more resonant than a violin. The viola pomposa is played on the arm and has a range from C'2 to A'5 (or even higher) with fingered notes. Using harmonics, the range can be extended to C'7 depending on the quality of the strings. It should not be confused with the violoncello or viola da spalla.
Not to be confused to violoncello piccolo (read Paulinyi, 2012. In English: Paulinyi, 2010).
Among the late Baroque and early Classical composers who used the instrument are Johann Sebastian Bach (1685-1750); Suite in D Major, BWV 1012 (designated "a cinq cordes"), Georg Phillip Telemann (1681-1767; two sections of Der Getreue Musikmeister), Johann Gottlieb Graun (c. 1703-1771; a double concerto with flute), Christian Joseph Lidarti (1730-1795; at least two sonatas).
By 1800, the instrument was used by principals of major orchestras, although no written scores were published in that century, apart from antiquarian or modernized editions (one of the Lidarti sonatas, heavily edited and with an added cadenza, was republished around 1904).
Late in the twentieth century, several contemporary composers independently rediscovered its potential because of the development of the new synthetic strings, more stable and cheaper than the gut ones. Recent music for the instrument includes works by Justin E.A. Busch, Harry Crowl, Rudolf Haken, and Zoltan Paulinyi.