A 5-string viola is a viola with an added high E string, extending its range and tonal possibilities, while maintaining the familiar viola tuning of C, G, D, and A.
Note: 5-string violas are tuned exactly the same as 5-string violins. Many players feel that the larger viola size is best for maximizing the tone and power of the lowest (C3) string. 5-string violins are far more common than 5-string violas.
D Rickert Musical Instruments 