Harmonic analysis of Chopin's Prelude #8

Measures 7 - 8

Measures 7 and 8 are a variation of measures 3 and 4. The melody is transported an ascending minor 3^rd. The harmony in the first 2 beats is exactly the same (a minor 3^rd higher), but from then on Chopin moves chromatically the dominant 7^th chords. Each 7^th chord can be analyzed as the German 6^th of the next chord:

The last chord will be used as a German Sixth chord. It will resolve into a Bb tonic in 2^nd inversion.

The series of dominant chords moving chromatically can also be explained using the tritone substitution concept from jazz theory. Tritone substitution is the substitution of a dominant chord by another dominant chord that shares the same tritone and that is at the distance of tritone from the first chord. For example, G7 can be substituted by Db7 because they both share the same tritone (B - F and F - Cb):

If you use tritone substitution on every other chord on a cycle of fifths progression you get a series of chromatically descending chords just like in Chopin's prelude: