Viola
The viola is a bowed string instrument. It is the middle voice of the violin family, between the violin and the cello. The casual observer may mistake the viola for the violin because of their similarity in size, closeness in pitch range (the viola is a perfect fifth below the violin), and nearly identical playing position. However, the viola's timbre sets it apart: its rich, dark-toned sonority is more full-bodied than the violin's.
The viola is similar in material and construction to the violin but is larger in size and more variable in its proportions. A "full-size" viola's body is between one and four inches longer than the body of a full-size violin, with an average length of about 16 inches. Unlike the violin, the viola does not have a standard full size. The body of a viola would need to measure about 21 inches long to match the acoustics of a violin, making it impractical to play in the same manner as the violin.
The viola is generally strung with thicker strings than the violin. This, combined with its larger size and lower pitch range, results in a tone which is deeper and more mellow. The thicker strings also mean that the viola "speaks" more slowly than its soprano cousin. Practically speaking, if a violist and violinist are playing together, the violist must begin moving the bow a fraction of a second sooner than the violinist to produce a sound that starts at the same moment as the violinist's sound. The thicker strings also mean that more weight must be applied with the bow to make them speak.
The viola plays an important role in chamber music. Mozart succeeded in liberating the viola somewhat when he wrote his six string quintets, which are widely considered to include some of his greatest works. The quintets use two violas, which frees the instrument (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 two solo instruments, viola and violin, are equally important.
The viola is also an important accompaniment instrument in Hungarian and Romanian folk string band music, especially in Transylvania. Among the great composers, several preferred the viola to the violin when playing in ensembles, the most noted being Ludwig van Beethoven, J.S. Bach and Wolfgang Amadeus Mozart. Numerous other composers also chose to play the viola in ensembles.
Amplification and equalization can make up for the weaker output of a violin string tuned to notes below G3, so most electric instruments with lower strings are violin-sized, and as such, are called "violins." Comparatively fewer electric violas do exist, for those who prefer the physical size or familiar touch references of a viola-sized instrument. John Cale, formerly of The Velvet Underground, is one of the more famous users of such an electric viola, who has used them both for melodies in his solo work and for drones in his work with The Velvet Underground (e.g. "Venus in Furs").