The violin is the most commonly used instrument in orchestra, and in a string quartet, the first violin is responsible for carrying the melody. The first violin plays the highest notes and the quickest passages. Since the first violin plays the most difficult runs, the musician playing this instrument is the leader of the group, and often is the most talented musician. The second violin supports the first violin, like a backup singer, and usually plays in a lower range than the first violin. This helps provide a fuller sound to the music.
Slightly larger in size than violins, and with a deeper pitch, violas are used to provide mid-range harmonic support between the violins and the bass instrument. While the viola has traditionally played less exciting parts than the violins, such as doubling the passages of the violins in a lower octave, recent composers have written pieces for the viola featuring solo leads and fluttering runs.
The cello is larger than the violin and viola, and provides a deep, rich, mellow sound, much like the tenor voice in a choir. The cello is the bass of the traditional string quartet, outlining the harmonic basis of a piece as the violins and violas play above and around it. At times, a double bass is often substituted for a cello in a string quartet. A double bass is even larger than a cello, about six feet high, and is the string instrument with the lowest pitch. The double bass provides a heavy bottom to the music, laying a foundation over which the higher stringed instruments can play.