String Quartet Number 19
From Memory Alpha, the free Star Trek reference
The String Quartet #19 in C major is a musical work composed by Wolfgang Amadeus Mozart in the late 18th century.
The first movement (Adagio-Allegro) was performed during a Mozart recital in honor of Vulcan ambassador Sarek aboard the USS Enterprise-D in late 2366. (TNG: "Sarek")
