A string quintet is a piece written for five bowed string instruments, usually two violins, commonly two violas and a cello, but sometimes a second cello or a double bass in place of the second viola. A string quintet is also an ensemble that performs such works. Mozart wrote six string quintets and they are widely regarded as some of his most outstanding pieces of chamber music. Schubert's Quintet in C for two violins, a viola and two cellos was his final instrumental composition and is considered to be one of his greatest achievements.