In Chopin's Nocturne in Bb minor, Op. 9, No. 1, m. 19, b. 4-6, we have a left-hand arpeggio comprising, in order, Db2 - Ab2 - Db3 - Abb3, while the right hand plays Fb.
This harmony sits between Db major on beats 1–3 and Ab7 over a Db pedal tone in the next measure.
Example mm. 19–20
There are at least two interpretations for this:
- Db minor with a non-harmonic Abb3 as a chromatic passing tone between the preceding Ab3 and upcoming Gb3.
- Gdim7 (i.e., Abbdim7), over an Ab pedal tone, as a leading-tone chord moving to Ab7.
- Something else...?
In functional harmony terms, how is this chord/passage best interpreted, and why?