Composed while the teenaged Mozart was in Milan, this quartet holds traits that reveal that Mozart was simultaneously working on an opera (Lucio Silla). The first movement is full of parallel thirds and sixths in the violins, intervals common in vocal duets. But perhaps this opening Allegro demonstrates a more instrumental nature through its primary, not particularly vocal theme, derived from ascending and descending C major scales. It's a bright but fairly gentle tune, graced with several trills in the first violin. A less important idea, tossed back and forth antiphonally between the two violins, serves as a bridge to a third, more singing melodic segment, beginning with the first violin playing something resembling a bird call. After a repeat of this exposition, Mozart subjects the material to a very brief development, with interesting work distributed among the four instruments, followed by the usual recapitulation of the first section.
The Andante shifts to C minor, and more than ever evokes an opera aria, thanks particularly to the first violin's plaintive line. This proceeds for the most part over a gently rocking accompaniment, aside from periodic episodes in which all four instruments take the theme in unison. The overall mood more than individual themes seems most important here, thanks to Mozart's rich and sometimes arrestingly modulating harmonies and that rocking rhythm not too far from the barcarolle, which would develop early in the following century.
The third and final movement -- there's no minuet in this quartet -- is a brief Presto, a rollicking sonata-rondo hybrid with lively syncopations and brief excursions into the minor mode. It's wrapped up with a tiny but stimulating coda, consisting of a crescendo (not yet a common technique) designed to bring Mozart's surprised audience to its feet.