I have a request from user @patrick91 to review this work, so here's my best attempt thus far:

It is impossible to discuss K. 465 without discussing its bequeathed namesake: It's chromatic, 'dissonant' Adagio introduction. Due to the chromaticism, it's actually ambiguous at the start whether the prevailing mode of the piece is going to be major or minor. At the very start, we get a solitary, repeating C in the cello. Promising. This is a 'C major' quartet, after all! So we expect a major chord...perhaps a minor chord? Lower down our list of expectations might be a B-flat as part of a C7 creating a 'quiescenza' voice-leading schema as in the beginning Mozart's E-flat Piano Quartet K. 493. But instead, the viola gives us...A-flat! Then the second violin gives us E-flat, which isn't too surprising since that completes a triad. However, what we get next is so shocking that connoisseurs — contemporary and well into the nineteenth century — tried to edit the passage so as to tame its dissonance level: a cross-relation created by the first violin on A-natural just as the A-flat in the viola steps down to G! Even Haydn, who was no stranger to the avant-garde or the art of surprise, was discomforted by this level of tension so early in a piece. Nevertheless, the dissonance is deliberate, and beautifully so. What we are witnessing is actually the accumulation of multiple chromatic lines in imitative polyphony. This opening has been written about extensively and is transformative in music history.

These concepts of imitative polyphony and chromaticism pervade throughout the entire piece. There are too many examples to list them all, but listen to, for instance, in the second movement, when an ostinato starts in the 'cello and the cannon that develops above it! The second time around is even more extraordinary. The whole movement has an interesting form; technically it's sonata form without repeats or development section (sometimes wrongly called 'sonatina form'), but the recapitulation has so much harmonic and contrapuntal expansion as to consist of a development section unto itself. The transition between movements two and three is very clever; the turn at the very end of the Andante (which occurs throughout the movement) becomes the opening chromatic gesture of the minuet.

The finale is full of the counterpoint, thematic unity, and effortless chromaticism that make Mozart so exceptional. The number of modulations and the rate at which they occur in the development has your head whizzing. I'd say it's reminiscent of the development in the fourth movement of Mozart's G minor Symphony no. 40 K. 550, but this quartet came first, so it's vice versa!

As with most of the mature works of Mozart, there is more to say than I can ever say here. Listen and discover the gems in all of Mozart's 'Haydn' quartets for yourself!

Also read: https://www.brentanoquartet.com/notes/mozart-quartet-k-465-dissonance/

Wolfgang Amadeus Mozart String Quartet No. 19 in C Major (Dissonance), K. 465