Chopin's Prelude #4 (measures 17 - 25)

Measures 17 to 25 have some interesting chord progressions.

In measure 20 - 21, Chopin uses a deceptive cadence (V - VI) then adds a Bb to the C major chord. This note create a C dominant 7^th chord but the Bb is used as chromatic appoggiatura and resolves to a II degree chord.

In measure 23 a German augmented 6^th chord is used with the augmented 6^th in the bass. In many editions it appears spelled as a dominant 7^th (C - E - G - Bb) just as the chord used in measure 21.