The 'alto' voice among the family of string instruments or 'chordophones.'
"...the viola was long overshadowed by the greater technical convenience of the smaller instrument. From the late 1700s, composers began to exploit the viola's characteristically mellow tone color..." (from 'Musical Instruments of the World' Bantam Books)
At between 16 to 18 inches long, the viola is slightly larger (by 3 inches) than the violin. The pitch from this instrument "should be half as long again as the violin," considering their relative sizes.
Heard to best advantage in pieces for string ensembles, the viola reached its best moment beginning with Haydn's and Mozart's string quartets in the late XVIII century. The instrument achieved even greater heights immediately afterwards with Beethoven's challenging quartets for strings, as would be the case during the Romantic era, with more from Schubert (Qt. No.14 in D minor; No.15 in G); by the early XXth century, Bela Bartók's own thrilling set of quartets would ice the cake.
In an orchestral setting, it would not be too venturesome to say that the viola part in Mozart's 'Sinfonia Concertante for violin and viola, K.364' truly merits consideration as a desert island pick.
