A string quintet is a musical composition for five string players. As an extension to the string quartet (two violins, a viola, and a cello), a string quintet includes a fifth string instrument, usually a second viola (a so-called "viola quintet") or a second cello (a "cello quintet"), but occasionally a double bass. The form was standard in 17th century Italy and can be seen as early as 1607 in Claudio Monteverdi's opera, L'Orfeo. Most famous of the cello quintets is Franz Schubert's Quintet in C major. Antonín Dvorák's Quintet Op. 77 uses a double bass, and Mozart's famous Eine kleine Nachtmusik may be performed with this instrumentation (the double bass being optional). Alternative additions include clarinet or piano (see clarinet quintet, piano quintet); and other closely related chamber music genres include the string quartet (much more common), the string trio, and the string sextet.