Compases 17 a 34

We propose to analyze this section in the key of Db Lydian. The constant use of the note G natural (marked in red) suggests this analysis.

Measure	Comments
17	We could analyze this measure as being in Db major. The G could be the third of the dominant chord of Ab (Eb7, Ab's dominant chord). Yet, the next measures do not support this interpretation.
18	We would expect this chord to use a Gb of a Db's dominant chord instead of G natural.
19 - 20	These measures are a variation of measures 17 and 18. The G natural is now also used in the right hand.
21	A half-diminished seventh chord resolves to G minor's dominant chord in the second beat.
22 - 24	These measures are clearly in the G minor key, but The D7 dominant chord in measure 24 will resolve to Db. Jazz theory would explain the D7 as a tritone subtitution. Traditional theory would say that D7 is an augmented sixth chord.

Chopin repeats measures 17 to 23 and adds an extra measure (in red). This times, he stays in G minor to end the prelude:

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