The viola is a bowed string instrument of the violin family. It has four strings tuned C, G, D, A, and serves as the middle voice between the cello (tuned an octave below it) and the violin (tuned a perfect fifth above it). In construction and shape it is similar to the violin, albeit larger in size. This, coupled with the fact that it usually employs heavier strings, makes for a warmer and deeper sound. The particular register of the viola makes it one of the few instrument with music mainly written in the alto clef. The viola has a established role as part of the string section of a standard orchestra. Though historically -and due to its register and tone- it has served accompaniment purposes, the 20th century saw an increased interest in the viola. Many specialized players emerged, and a substantial amount of solo repertoire was produced.