The viola is a bowed string instrument. It is the middle voice of the violin family, between the violin and the cello.
In early orchestral music, the viola part was frequently limited to filling in harmonies and little melodic material was assigned to it. The viola plays an important role in chamber music. Mozart succeeded in liberating the viola when he wrote his six string quintets, some of which are considered to be his greatest works. The quintets use two violas, which frees them (especially the first viola) for solo passages and increases the variety and richness of the ensemble. Mozart also wrote for the viola in his Sinfonia Concertante in which the solo viola and violin are equally important, a set of two duets for violin and viola, and the Kegelstatt Trio for viola, clarinet, and piano.
The viola is sometimes used in contemporary popular music, mostly in the avant-garde. John Cale of The Velvet Underground used the viola, as do some modern groups such as alternative rock band 10,000 Maniacs, folk duo John & Mary, Defiance, Ohio, The Funetics, Flobots, Beethoven's 5th, British Sea Power, Hangedup and others. Jazz music has also seen its share of violists, from those used in string sections in the early 1900s to a handful of quartets and soloists emerging from the 1960s onward. It is quite unusual though, to use individual string instruments in contemporary popular music. It is usually the flute or rather the full orchestra appearing to be the favoured choice, rather than a lone string player. The upper strings could be easily drowned out by the other instruments, especially if electric, or even by the singer.