to the official website of Tabea Zimmermann
The viola is seemingly just a big violin but tuned a fifth lower. In reality the two instruments are worlds apart. They both have three strings in common, the A, D, and G string. The high E-string lends the violin a powerful luminosity and metallic penetrating tone which is missing in the viola. The violin leads, the viola remains in the shade. In return the low C-string gives the viola a unique ascerbity, compact, somewhat hoarse, with an aftertaste of wood, earth and tannic acid.
György Ligeti in the preface for his Sonata for Viola solo (1991–1994)