The viola’s tone is thicker and darker than the violin which makes it perfect for providing resonant solo passages in the middle of the orchestral range.
Traditionally the viola was used to fill in the harmonies between the expressive cello and violin parts. Since the 19th century viola writing is just 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 its lowest note is a perfect 5th lower than the violin.