Show off your theory chops with my weekly challenge! You’ll find a new question here every Monday. Please comment to post your reply.
This week’s challenge:
This is the end of Chopin’s powerful Nocturne in C minor, Op. 48, No. 1 The excerpt begins with him shaping up to have a typical authentic cadence, but it’s shattered when we get to the red rectangle. What is the harmony there, and where does it lead us? Hint: think about how this chord should be spelled, and notice how it’s actually spelled!

Reply to post your answer, and check back on Friday, October 22 to see if you’re right!
ANSWER for 10/18/21

Chopin has taken the German augmented sixth chord in C minor and reinterpreted it as the dominant of the Neapolitan key (bII). Below at left is a typical authentic cadence preceded by a German 6th; the F sharp in the bass resolves up to G (FI up to SO in solfege) as it should. But at right Chopin has respelled that F sharp in the bass as G flat. Now the chord is no longer a German 6th, but a plain old Ab dominant seventh in third inversion, resolving to D flat Major in first inversion (N6 or bII6).

This reinterpretation is possible because a German 6th is enharmonic with a dominant seventh chord. Until you see how they’re spelled–or hear how they resolve differently–they both sound the same! Chopin’s reinterpretation of the German 6th creates a wonderfully jarring deceptive cadence, taking us to the remote world of the Neapolitan before heading back home.
With Breaking Barlines you learn music theory the right way: fun, holistic, and with a personal touch! Have a look at the complete Breaking Barlines Course! Then sign up for a monthly subscription for full access to all video lessons, worksheets, and answer keys. New lessons are always being added, so stay tuned!