The viola is the alto or tenor instrument of the violin family.  It is bigger than a violin, and lower-pitched, but otherwise it looks similar.  The strings are tuned to C (below middle C), G, D, A which are all one fifth lower than the corresponding strings on a violin.   Though its lowest notes are fairly low, they do not have a lot of power, as the viola has a  small body, unlike its bass cousin, the cello. Its tone is somewhat rougher than the violin's, and it has not been used as a solo instrument to any significant extent.

However, there are a few important works written for the viola. Berlioz wrote his tone poem "Harold in Italy" for viola and orchestra, and Mozart wrote his "Sinfonia Concertante" for violin and viola with orchestra.

In the orchestra, the violas form a vital section of the strings, as they contribute the "tenor" part to the harmony. In string quartets, where there are two violins, one viola, and one cello, the viola becomes a lot more prominent.  Here, its unique tone colour is easily distinguishable from the sweeter but more strident tone of the two violins.