Harmonization is the beating heart of a prelude. In the 18th and 19th centuries, chord progressions formed the groundwork of the preluding discipline. Such a chord progression could be worked out in advance or improvised in the moment. If there were such a thing as a 'map' that could guide the performer, then the harmonized scale (also called the 'rule of the octave') was the most advisable.
1. SCALE HARMONIZATIONS
Around 1840, Joseph Zimmermann (1785-1853), the famous pianist and teacher at the Conservatoire National Supérieur de Musique de Paris, gave advice on the use of harmonic progressions for preluding. One of the most important progressions he describes in his publication Encyclopédie du Pianiste Compositeur (part III, p. 19) is the rule of the octave:
We understand that in preluding we are not bound to make this scale heard in its entirety by going up and down; one can use fragments of it, and vary it in a thousand ways. When you have familiarized yourself with the rule of the octave, your fingers will have acquired an instinctive habit of bringing to each degree of the scale the right chord, which is a huge advantage.
Chord progressions such as the rule of the octave were in use throughout the 18th and 19th centuries. Carl Philip Emmanuel Bach (1714-1788) gives the following advice on harmonization of a vorspiel (introduction or prelude) in his book Versuch über die wahre Art das Clavier zu spielen (p. 327-328):
The shortest and most natural approach, which can also be used by keyboard players with few skills in playing introductions, is as follows: One builds a foundation based on the ascending and descending scale of the prescribed key with a variety of figured bass signatures (a), by interpolating a few half steps (b), by arranging the scale in or out of its normal sequence (c), and performs the result in broken or sustained style at a suitable pace. A tonic organ point is convenient for establishing the tonality at the beginning and end (d). The dominant organ point can also be introduced effectively before the final chord (e).
2. THE BASIC CADENZA AS A STARTING POINT
In his publication The Art Of Preluding the Austrian pianist, composer and teacher Carl Czerny (1791-1857) considers the basic cadenza as the shortest prelude, but on the next page, he gives examples of chord progressions that are extended to six or more chords.
3. CHROMATIC SCALE HARMONIZATION
By adding chords on top of each semitone step of the scale, secondary dominant chords can be created. This increases the number of chord combinations. It is important not to play this series of chords consecutively, but out of order. The arrows indicate that certain chords move in a specific direction. Playing the chords of the chromatic scale harmonization in random order, taking into account the playing direction of the secondary dominants and the subsequent chords, is an ideal exercise in mastering harmonizations and creating 'harmonic storylines'. One can practice this in all keys. While playing, it is important to have the harmonic functions in mind: tonic, dominant, subdominant and secondary dominant.
4. UNIVERSAL PROGRESSIONS FOR STUDYING
Studying commonly used chord progressions like the rule of the octave provides two useful universal progressions that can serve as a starting point for preluding: one showing an upward motion and another a downward motion. Transposing these progressions to other keys and applying the exploration techniques (see section 'explorations') can be a first big step towards preluding in a historical context. These progressions can be expanded, changed and personalized according to the pianist's preferences.
Once one is familiar with the upward and downward progressions, the chords can play be played in or out of their normal sequence, again keeping in mind the harmonic functions.
upward and downward motion
5. 'FORTSCHREITUNGEN' or 'MARCHES D'HARMONIES'
Another progression, here called circular motion, can often be found in preludes and is an example of the so-called 'Fortschreitungen' or 'marches d'harmonies', a repetition of chords at regular intervals in parallel motion.
Different versions of the circular motion below can be found in Cours de Composition Musicale written by the famous composition teacher Anton Reicha (1770-1836).
An interesting exercise that combines different types of 'Fortschreitungen' can be found on page 14 in Czerny's Studien zur praktischen Kenntniss aller Accorde des Generalbasses, Op.838.