Violas. - In effect, the viola is an alto violin. It looks pretty much like a violin only slightly larger. Its tone is different, too, softer, rounder and less brilliant. The viola has its own clef - the alto clef - and it plays in the alto and tenor registers of the voice much of the time. It was developed at about the same time as the rest of the modern string family, in the 16th and 17th centuries.
Some early builders, like the expert Cremona builders Amati and Salo, tried to get more sound out of the viola by building it larger. But it can't be much larger than it is and still be played comfortably on the arm. Modern builders still experiment occasionally with its design, but many people like the viola just as it is.
As solo instruments, violas have been somewhat slighted by composers who tend to want to write for the more flashy violin. The viola can be somewhat hard to hear over the orchestra unless the composer is very skillful in writing the orchestra parts. Before 1750, there was really no solo literature for the viola at all. Some composers - including Bach, Haydn, and Mozart - loved to play the viola in string ensembles. They liked its sound and they enjoyed playing the interesting inner parts.
Mozart was one of the first composers to treat the viola seriously, writing really interesting parts for it and exploring its tonal possibilities, even writing a concerto part for it. Beethoven followed his lead, and in the 19th century Berlioz, with his fine ear for orchestral effects, wrote a beautiful viola solo in a piece called Harold in Italy.
In recent years, composers are taking more notice of the viola's special, deeper voice,and more concertos are being written for the instrument - some inspired by fine solo viola players like Russia's Yuri Bashmet and America's Pinchas Zukerman. Russian composer Sofia Gubaidulina recently wrote a double viola concerto specifically for the New York Philharmonic's first-desk viola players, Cynthia Phelps and Rebecca Young.