The Viola is the middle voice between the cello and the violin. The Viola was invented in the 1500's however, it remains a mystery to who actually invented it.
The 20th century has seen some amazing pioneers of the viola some of these where Ernst Wallfisch, Walter Trample, Lionel Tetris, Maurice Vieux, Paul Hindemith, Frederick Riddle, William Primrose, Cecil Aronowitz, Lillian Fuchs, Theophile Laforge, Vadim Borisovsky.