Not simply a big violin, the viola has its own distinct qualities and serves an essential function in orchestral music. Its tone is thicker and darker than the violin which makes it perfect for providing resonant solo passages in the middle of the orchestral range and the perfect antidote to the penetrating, crystal quality of the violin. It also blends particularly well with the clarinet and bassoon. Traditionally the viola was used in uninteresting ways simply to fill in the harmonies between the expressive cello and violin parts, but this is no longer the case. Violas can be every bit as agile as the other strings and since the 19th century viola writing has been every bit as demanding as that for the other string instruments.
The viola is the alto voice of the string family. It is larger than the violin, and it´s lowest note is a perfect 5th lower than the violin. A typical symphony orchestra will have twelve violas seated two to a 'desk' for a large orchestral work.
Photograph courtesy of Bishop Strings