The chord circled in red is what I am asking about. I know that in the measure before, there is a diminished seventh chord. I'm certain that this is a dominant function chord(or at least, locally to the measure, it has dominant function(tritone is the biggest giveaway here)). This, along with 4 notes in the chord, narrows things down to:
- Dominant 7th(secondary or primary)
- Half Diminished 7th(secondary or primary)
- Diminished 7th(secondary or primary)
- Augmented Sixth
- Tritone substitution
Granted, the tritone substitution is pretty much nonexistent in Chopin's era(closest you get in notation is an unusually resolved augmented sixth chord), so that is out.
What are the notes in the chord? They are these notes:
Db, G, Bb, Ab
Or at least, that is what makes the most sense to me. But if I try to form a chord from these notes that Chopin would have used, I get nothing. Closest I get is a sixth chord(in the sense that there is an added sixth). Chopin wouldn't have used a sixth chord, at least, not a major or minor sixth chord. Could this be an augmented sixth? Well, these are the sixth intervals that are possible, including the possibility of F being a chord tone:
Db-Bb - Major Sixth
Bb-G - Minor Sixth
No augmented sixths in sight. Could this be a dominant seventh chord? No. Only 1 chord seems to fit, G halfdim7. This resolves to Ab major as expected. And this becomes part of a sequence, both melodically and harmonically.
So, the chord in question would be viiø7/IV, right, with the preceding chord being vii°7/IV?