19 in C major, K. 465, is a quartet by Wolfgang Amadeus Mozart, nicknamed "Dissonance" on account of the unusual counterpoint in its slow introduction.

Mozart continues, "They are, it is true, the fruit of a long and laborious endeavour..."[1]: 250  In these quartets he deviated from his usual practice of short scoring Hauptstimmen (main voices) and filling in the rest later.

[7] When the Allegro begins, the cello is silent, and the viola has taken up its eighth note Cs, playing them an octave higher and much more ebulliently than the opening bars.

[10]: 57  Alfred Einstein writes of the coda of this movement that "the first violin openly expresses what seemed hidden beneath the conversational play of the subordinate theme".

Mozart evokes Haydn's witty deployment of rests, which creates textural variety and contrasts the melodic material.

The development is as harmonically audacious as the piece's introduction as it modulates through a circle of fifths in minor keys before returning to the main theme.

[10]: 59f The string quartet is one of Mozart's most analyzed compositions and has a long history of musicological debate that began almost immediately upon its publication.

The correspondent's letter was written on January 29 from Vienna, and reported on Haydn's visit to the city as well as Mozart's plans to travel to Prague and Berlin.

The writer lamented the waste of Mozart's prodigious keyboard talent on composition and quipped, "...his new Quartets for 2 violins, viola and bass, which he has dedicated to Haydn, may well be called too highly seasoned-and whose palate can endure this for long?

"[12] Two years later, in the same periodical (now published in Copenhagen), Mozart's complexity was praised, "...his six quartets for violins, viola and bass dedicated to Haydn confirm it once again that he has a decided leaning towards the difficult and the unusual.

"[1]: 349  By 1799, an anecdote from Constanze Mozart was being repeated in the pages of Allgemeine musikalische Zeitung (AmZ) that the Italian printer sent the engravings back to Artaria because he assumed the notes were errors.

[14] In his analysis of the quartet, Sarti called the violin's opening dissonance "execrable" and accused the composer of having "ears lined with iron".

Sarti also analyzed K. 421 with his poison pen and concluded, "From these two examples it may be perceived that the author (whom I neither know nor wish to know) is nothing more than a piano-forte player with spoiled ears (!).