The prelude: from Bach to Chopin, Debussy and the opera prelude

Definition

The prelude began as an introductory piece, often improvised, with which the musician “tried out” the instrument and established the key before a more important piece (a fugue, a suite, a liturgical service). Over time it became an autonomous genre: short character pieces, usually based on a single figuration or a single mood, often gathered in cycles that run through all the keys.

The main types

Unmeasured prelude (prélude non mesuré): seventeenth-century French harpsichordists (Louis Couperin, d'Anglebert): written in whole notes without bar lines, the performer decides rhythm and duration.
Prelude and fugue (Bach): the free prelude (arpeggio, toccata, invention, aria) followed by a fugue in the same key. The Well-Tempered Clavier: Book I (1722) and Book II (compiled around 1742), 24 pairs each in all the major and minor keys, in chromatic order (C major, C minor, C# major…).
Suite prelude: introduction to the suite (English Suites, Cello Suites).
Prelude as a character piece: Chopin, Scriabin, Rachmaninoff, Debussy.
Opera prelude (Vorspiel): orchestral introduction freer than the overture, leading directly into the first scene (Wagner, Lohengrin, Tristan und Isolde; Verdi, La traviata).
Chorale prelude for organ: elaboration of a chorale melody (Bach, Orgelbüchlein).

Chopin's cycle: 24 Preludes, Op. 28 (published in 1839)

Ordered by the circle of fifths, each major followed by its relative minor: C major, A minor, G major, E minor, D major, B minor…
Durations from under a minute to a few minutes; each one is a single idea (an aria, a figuration, a chorale).
No. 4 in E minor: nearly motionless melody over left-hand chords that descend chromatically.
No. 7 in A major: 16 bars, mazurka rhythm.
No. 15 in Db major (“Raindrop,” nickname not by the composer): repeated note A♭/G♯ throughout the piece, central section in C# minor.
No. 20 in C minor: chorale of chords in 13 bars.

Other cycles

Scriabin, 24 Preludes, Op. 11 (same order as Chopin).
Rachmaninoff: Prelude in C# minor, Op. 3 No. 2 (1892) + Op. 23 (10) + Op. 32 (13) = 24 preludes in all the keys.
Debussy, Préludes, Book I (1910) and II (1913): 12 + 12; the title is written at the end of each piece, in parentheses, so as not to condition the listening (La cathédrale engloutie, La fille aux cheveux de lin, Voiles on the whole-tone scale).
Shostakovich, 24 Preludes, Op. 34 (1933) and 24 Preludes and Fugues, Op. 87 (1950–51).

Typical analysis of a “figuration” prelude (Bach, Prelude in C major BWV 846)

Each bar is an arpeggiated chord with the same pattern (5 notes up, then repetition).
Reduce each bar to a five-voice chord: C – Dm7/C – G7/B – C – Am/C – D7/C – G/B – Cmaj7/B – Am7…
Follow the voice leading: each voice moves by step, as in a chorale.
Find the dominant pedal (from about bar 24) and the final tonic pedal with the arpeggio dissolving.
Conclusion: it is a five-voice chorale “spread out” in time.

For a producer

The prelude is the model of the intro and the interlude of an album: a single, brief idea that establishes key and atmosphere and leads into the main piece. Bach's prelude teaches how to make a continuous arpeggio interesting: voice leading between chords and a final pedal.

Fugue: formal plan, Étude, Nocturne, Inner voices and hidden lines, Leitmotif and music drama (the Tristan prelude).