The general structure of a string quartet is:
Violin I
Violin II
Viola
Cello
The Violin I plays the Soprano parts, the Violin II plays the Alto parts, the Viola plays the Tenor parts, and the Cello plays the Bass parts. The two most important parts of a string quartet are the Violin I and Cello. The Violin I and the Cello the melody plays the counterpoint, while the Violin II and Viola parts are filler parts that are there to make the piece sound nicer.