The viola is a string instrument that is bowed or played with varying techniques. It is slightly larger than a violin and has a lower and deeper sound. Since the 18th century, it has been the middle or alto voice of the violin family, between the violin (which is tuned a perfect fifth above) and the cello (which is tuned an octave below). The strings from low to high are typically tuned to C3, G3, D4, and A4.
In the past, the viola varied in size and style, as did its names. The viola often plays the “inner voices” in string quartets and symphonic writing, and it is more likely than the first violin to play accompaniment parts.
The viola was popular in the heyday of five-part harmony, up until the eighteenth century, taking three lines of the harmony and occasionally playing the melody line. Music for the viola differs from most other instruments in that it primarily uses the alto clef. When viola music has substantial sections in a higher register, it switches to the treble clef to make it easier to read. The viola plays an important role in chamber music. Mozart used the viola in more creative ways when he wrote his six string quintets. The quintets use two violas, which frees them (especially the first viola) for solo passages and increases the variety of writing that is possible for the ensemble. Mozart also wrote for the viola in his, “Sinfonia Concertante”, a set of two duets for violin and viola, and the Kegelstatt Trio for viola, clarinet, and piano.