String instruments are chordophones, instruments that sound by the vibration of a string of a certain tension. The string section of the modern orchestra uses first and second violins, violas, cellos and double basses. A string trio consists of violin, viola and cello; a string quartet consists of two violins, viola and cello and a string quintet either of two violins, two violas and cello, as in the case of Mozart's work in this form, or of two violins, viola and two cellos, as in the case of Schubert's famous C major String Quintet and the Quintets of Boccheri. Other numbers and combinations of string instruments are possible in other ensembles.