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.
|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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