Measures 17 to 34

We propose analysing this section in the key of D-flat Lydian. The consistent use of the note G natural (marked in red) indicates this approach.

Measure	Comments
17	We could analyse this bar as being in D-flat major. The G could be the third of the dominant chord of Ab (Eb7). However, subsequent measures do not support this interpretation.
18	We would expect this chord to use a G-flat instead of a G natural, which is the dominant chord of a D-flat.
19 - 20	These measures are variations of measures 17 and 18. The G natural is now also used in the right hand.
21	The second beat sees a half-diminished seventh chord resolve to the dominant chord of G minor.
22 - 24	While these measures are clearly in the key of G minor, the D7 dominant chord in bar 24 will resolve to Db. According to jazz theory, the D7 chord is a tritone substitution. According to traditional theory, D7 is an augmented sixth chord.

Chopin repeats measures 17 to 23, adding an extra measure in red. This time, he remains in G minor until the end of the prelude:

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