Measures 1 to 6 - Theme in E flat major

The first bars present the theme on which the prelude is based. Below we analyse its main characteristics.

The notes in red structure the theme melodically and harmonically.

• Measures 1 to 3: E flat arpeggio (Eb - Bb - G).
• Measure 4: the A natural begins the modulation to B flat major.
• Measure 5: a II degree followed by a V affirms the key of B flat major.
• Measures 1 to 4: tonic pedal.

The picture below shows the motifs that make up the theme.

1. Motives 1 and 2 are repeated in each bar.
2. The harmonic tension of the first two notes of motive 2 leads to its third note.
3. Motive 3 is repeated with variations.

Measures 6 to 11 - Theme in B-flat major

• The theme is repeated in the key of B flat major.
• Another modulation by an ascending fifth takes us to F minor.
• The harmonic plan is the same, but there are some melodic variations.
• We show bars 3 to 6 with the varied notes marked in red. The notes in blue have been transposed down an octave.

Measures 11 to 14 - Development

When F minor is reached, an episode or development based on the theme begins. It consists of a circle of fifths progression (F - Bb - Eb - Eb - Lab - D - G - C) leading to C minor.

• We have highlighted motives 1, 2 and 3.
• On the harmonic function of motive 2:
• Bar 11: minor seventh chord in first inversion, C and Eb lead to D.
• Bar 12: augmented E flat chord with major seventh in first inversion, B natural and D lead to C.
• Bar 13: minor seventh chord in first inversion as second degree of C minor, A and C lead to B.

Measures 14 to 19 - Theme in C minor

The theme returns, this time in C minor. As expected, we modulate again by ascending a fifth to G minor.

Measures 19 to 23 - Development

A circle of fifths progression (G - C - F - Bb - Eb) brings us back to Eb major.

Measures 23 to 25 - Development

E flat becomes the dominant of A flat major and leads us into this key:

Measures 25 to 30 - Theme in A-flat major

The theme is presented in A-flat major and modulates by an ascending fifth to E-flat major:

Measures 30 to 38 - Development

• Measures 30 to 33: an episode using a circle of fifths progression (Eb - Lab - D - G - G - C - F - Bb) leads to the dominant of Eb major.
• Measures 33 to 35: the dominant of E flat is emphasised in preparation for the climax.
• Measures 36 to 38: similar passage to measures 23 to 25, but this time it takes us to A flat minor.

Measures 38 to 40 - Development

• The climax of the prelude is in Ab minor.
• Bach insists on motive 1.
• Bar 39: The chord of F-flat major, VI degree of A-flat minor is used as a Neapolitan sixth of E-flat major.
• Bar 40: Bach's ingenuity surprises us once again, as the Neapolitan sixth resolves into an auxiliary dominant of the V degree of E flat major. The dominant is in a third inversion and has a suspended fourth.
• Measure 40 (third beat): we continue in E-flat major as if nothing had happened.

Final measures

We end with the theme in E flat major. Note the parallel fifths on the third and fourth beats of bar 46.

Listen to the Prelude