Preface
@11684 Gives a very good explanation in contrapuntal terms of how this passage works. It should be considered carefully. I'll give an explanation in functional analytic terms. Loosely speaking, one could say that @11684 gives a melodic explanation for the passage, and I offer a harmonic one.
TL;DR
Measures 21–22 are an expansion of D# minor; that is, the key of ii. They are the beginning of a ii V i progression leading to C# minor in m. 25.
Detailed explanation
In mm. 13 – 16, Chopin establishes a very clear ii V I progression in E major. (See Examples 1a and 1b, below.)
Measures 17–18 and the first half of 19 repeat mm. 13–14 and the first half of 15, but the second half of m. 19 signals a change of key back to C# minor. The F# minor chord that begins m. 19 is a pivot chord, serving as both ii of the old key (E Major) and iv of the new key C# minor. This sets up the V chord of C# minor (G#) in measure 20. (See Example 2, below.)
One way to confirm the transition to C# minor is to return to the main theme at this point: that is, cut measures 21–24, and just straight from m. 20 to m. 25.
Measures 21–24 serve to delay and strengthen the eventual arrival, in m. 25, of the C# minor tonic harmony. Measure 21 through the first half of measure 24 elaborates D# minor, the ii harmony, while the second half of measure 25 states the V chord, G#.
Measure 21 comprises two chords.
- The
B7 chord is both VII in C# minor and bVI (Cb augmented sixth chord) in D# minor.
- The
D# minor chord is ii in C# minor and I (or more accurately, cadential 6-4) in D# minor.
The sense of transition can be confirmed by playing the progression shown in Example 3.
The diminished chord is measure 22 is both a common-tone diminished seventh chord relative to D# minor (primarily) but also vii of C# minor (secondarily, as the passage emphasizes the D# minor chord).
Measure 23 repeats measure 21, but measure 24 returns to the augmented sixth chord, ow in root position (via voice exchange: see @11684's answer) and doubling as bVII in C# minor — another pivot chord. The bVII chord proceeds to the V chord of C# minor (Example 4), and then in measure 25 the main theme returns in C# minor.
Conclusion
Measure 21 through the first half of 24 are a prolongation of the D# minor (ii of C# minor) harmony, leading to the dominant chord in the second half of m. 24, and resolving to C# minor with the return of the main theme in m. 25.
Examples
Example 1a: mm. 13–16
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Example 1b: mm. 13–16 reduction
X: 1
T: Chopin Op. 66
T: mm. 13–16
K: C#m
M: C|
L: 1/2
V:V1 clef=treble
V:V2 clef=bass middle=D
%%score V1 | V2
[V:V1] [Fcf]2 | [DFAd] [EGBe] | [Fcf]2 | [FBdf] [GBeg] |
[V:V2]"ii"[A,CFA]2|"V"[B,FAB]"I"[E,B,EG] | "ii"[A,CFA]2 | "V"[B,FAB] "I"[E,B,EG] |
Example 2: Measures 19–20
The F# minor chord comprising the first half of m. 19 functions as ii in the key of E major, as previously, but also as iv of C# minor. The subsequent D# half-diminished chord is ii in C# minor (also vii in E major), leading to V of C# minor.
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Example 3: D# minor cadence extending measure 21
X: 1
T: Chopin Op. 66
T: mm. 21 with cadence added
K: C#m
M: C|
L: 1/2
V:V1 clef=treble
V:V2 clef=bass middle=D
%%score V1 | V2
[V:V1] [Bdfb] [^Adf^a] | [^A^^c^e^a] [DF^Ad] |
[V:V2] "bVI"[A,DFB] "C6-4"[^A,DF^A] | "V"[^A,^^C^E^A] "i"[D,F,^A,D] |
Example 4: bVII7- V[6-5] progression
Aldwell and Schachter (Harmony and Voice Leading, 2nd ed, 1989) give two examples of VII V i. Their example 24-19 (p. 389) gives a passage from Handel's third harpsichord suite, first movement, measures 11–16. The piece is in F major, and the harmony progresses from G minor to C7, seemingly setting up a ii V return to F. However, the C7 progresses to A7 and then to D minor.
The progression is also discussed in the context of voice exchange on pages 537–38.
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