The viola is approximately 2 inches longer than the violin (and wider proportionally), though there is greater variation in the size of the viola than that of the violin. Compared to the violin, the viola has a more resonant and mellower sound, especially in the lower registers. The viola is held horizontally beneath the chin and played in much the same way as the violin.
The viola reads music in the alto clef, and has 4 strings tuned in fifths: the C an octave below middle C is the lowest, with G, D and A above it - these are tuned exactly one fifth below the violin, and so one octave above the cello.
Use of the viola is almost completely limited to classical music, and even in that field it is not nearly so popular an instrument for solo pieces and sonatas as its cousins the violin and the cello. In orchestral music the viola part is frequently limited to the filling in of harmonies with little melodic material assigned to it. There are also very few viola concertos compared to the violin or cello. A rare example of a piece written before the 20th century which features a solo viola part is Hector Berlioz's Harold In Italy, though there are also a few Baroque and Classical concerti, for example those by Telemann and Carl Stamitz respectively.
In the 20th century, more composers began to write for the viola, encouraged by the emergence of specialised solo violists such as Lionel Tertis. William Walton and Bela Bartok have both written well-known viola concertos. One of the few composers to write a substantial amount of music for the viola was Paul Hindemith, who was a violist himself. Rebecca Clarke is a 20th century composer who wrote almost exclusively for the viola. However, the amount of music in the viola repertoire remains quite small, and violists often play arrangements of other pieces.
For some reason, violas and violists bear the brunt of the classical musical world's humorous derision, being the target of the musical equivalent of the blonde joke. (Drummers in popular music and trombonists in brass band music are the butt of similar jokes.) There is a commonly held (mostly erroneous) belief that one only becomes a violist when one has tried and failed to master the violin. J. S. Bach and L. van Beethoven are known to have expressed a fondness for playing the viola; of Bach, it is said that "As the greatest expert and judge of harmony, he liked best to play the viola, with appropriate loudness and softness." (C. P. E. Bach)
There are very few viola virtuosi, owing to the shortage of music for the instrument. Among the better known violists, from earlier in the twentieth century, as well as Tertis are Paul Hindemith, William Primrose and Walter Trampler, and from more recently, Yuri Bashmet, Kim Kashkashian and Tabea Zimmermann.
As could be expected, the viola also sees little use in popular music. It was sometimes part of popular dance orchestras in the period from about 1890 to 1930, and orchestrations of pop tunes from that era often had viola parts available. The viola largely disappeared from pop music with the changes of dance band orchestrations at the start of the big band era. Unusually, John Cale, a classically trained violist, played the instrument to great effect on both Velvet Underground albums he appeared on, The Velvet Underground and Nico and White Light/White Heat.
The term violist is not universally used in English, some players preferring viola player. This may be a US/UK divide or something more complex.