The original name of this instrument was "violoncello", but it is usually shortened to cello. Similarly, we say piano instead of "piano forte", as the keyboard instrument was originally called.
The cello is much larger than the viola and its strings are tuned an octave and a fourth below the strings of the violin: c - g - d - a. The cello player sits and braces the instrument between his knees.
The cello is the bass instrument of the string quartet.
