Professional violinists and violin teachers tend to be very comfortable playing and teaching viola on some level. However, the viola differs from the violin in some important ways.
In the family of orchestral string instruments, the viola occupies the middle register, comparable to an Alto in a choir. The viola sound is also darker in sound than the violin. The viola's darker and lower sound have much to do with the size of the instrument, which is slightly larger than the violin.
Another important difference between violin and viola is that viola music is mostly notated in alto clef. Requiring good note reading skills, it switches to the more common G clef only for the highest register.
In string quartets and orchestral symphonies, the viola plays the lines most suitable for the instrument--those in the middle register. As such, the viola--much like the alto in a choir--performs a very important role even if it is not quite as prominent as the higher violin parts featuring the main melodies. Nevertheless, a few composers since the 18th century have written solo concertos for the viola to spotlight its beautiful sounds.
More recent examples of viola concertos include those by twentieth-century composers Paul Hindemith, Béla Bartók, and William Walton.