Which clef is the music most commonly written in for a viola?

The clef that music is most commonly written in for a viola is the Alto clef. The viola is a stringed instrument slightly bigger than a violin, that has a sound that is lower than a violin's. The viola clef, or Alto clef, is also known as the 'C' clef because 'Middle C' falls on the center line that goes through the middle of the "puffy C".

Alto clef is between the higher Treble clef and the lower Bass clef. Instruments like the violin, that play higher notes, use the Treble clef; whereas lower sounding instruments, like the cello, use the Bass clef. Pianists use both Treble (right hand) and Bass (left hand) clefs. This is why the viola uses its own Alto clef.