To answer anyone's question on what exactly a viola is, a viola is the alto in the violin family, which evolved from the viol, an instrument with many strings that is bowed and held across the knees. They used to be called the "alto-tenor" violin. The viola we know today is the result of the gradual merging
of the alto and tenor violas over three centuries. Most musicians did not like to play the tenor viola because it was so large in size and difficult to play because of this. So, gradually, luthiers stopped making the tenor viola. The alto viola's neck was too short to play the music that was getting more difficult by the 16th century. So luthiers lengthened the neck of the alto viola in order for musicians to play a wider range of notes. The modern-day viola is about 1-3.5 inches longer than a violin, making them around 16 to 18 inches long. Today they are the only instruments that use the alto clef.
Violas are not nearly as famous as the violin or cello, and they probably never will be. There are very, very, few solo violists, especially compared to the amount of solo violinists. In the ordinary orchestra, there are about three violins to every viola, and sometimes more. Composers sometimes wouldn't even bother writing a viola part. Violas would end up playing the bass part. It wasn't until the opera "Orfeo," written in 1607, that violas began to become more important. Gradually, violas earned their own parts, and even get very important parts in pieces. Violas took a secondary role in the 17th and 18th centuries, even though musicians such as Mozart and Bach were both accomplished violists. It resurged with compositions like "Harold in Italy" in 1834 by French composer Hector Berlioz. Johnannes Brahms and Hector Schumann also wrote important pieces for violas