Why do violas commonly use the alto clef?

In musical notation, music is typically written in the clef that most closely corresponds to the range of the instrument's part. The viola is tuned a fifth lower than the violin, with its strings tuned to C–G–D'–A'.123

Because of this lower range, the viola uses the alto clef—a type of C clef where middle C is positioned on the middle line of the staff. This placement allows the viola's music to be written efficiently within the staff without requiring excessive ledger lines, which would be necessary if the instrument were notated in the treble clef used by the violin. Consequently, the viola is the only standard orchestral string instrument that regularly utilizes the alto clef.412