Harmonization is the beating heart of a prelude. In the18th and 19th centuries, chord progressions form the groundwork of the preluding discipline. Such a chord progression can be worked out in advance or improvised at the moment. If there is such a thing as a 'map' that can serve for guidance, then the harmonized scale (also called the 'octave rule') is the most advisable.
Around 1840 the famous pianist and teacher at the Conservatoire National Supérieur de Musique de Paris Joseph Zimmermann (1785-1853) gave advice on the use of harmonic progressions for preluding. One of the most important progressions he described is the octave rule, of which he states in his publication Encyclopédie du Pianiste Compositeur (part III p19):
We understand that by 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 octave rule, your fingers will have acquired an instinctive habit of bringing to each degree of the scale the right chord, which is a huge advantage.
Practice sheet including cadenzas.
Chord progressions such as the octave rule are in use throughout the 18th and 19th century. 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 (page 327-328):
Following are the briefest and most natural means of which a keyboard player, which even keyboard players with few skills can use to play, is this: to play out of the ascending and descending scale of the prescribed key, with a variety of figured bass signatures (a), interpolate a few halfsteps (b), arrange a scale in or out of its normal sequence (c), and perform the resultant progressions in broken or sustained style at a suitable pace. A tonic organ point is convenient for establishing the tonality ant the beginning and end (d). The dominant organ point can also be introduced effectively before the close (e).
In his publication The Art Of Preluding the Austrian pianist, composer and teacher Carl Czerny (1791-1857) considers a progression of two chords already as a prelude, but in e next step his examples of basic progressions explore a longer series of chords.
UNIVERSAL PROGRESSIONS FOR STUDYING
Studying commonly used chord progressions provides two useful universal progressions that can serve as a starting point: one showing an upward motion and another a downward motion. Transposing these progressions to other keys, and combining them with explorations (see platform '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 choice.
upward and downward motion
As soon as the upward and downward progressions are known one can arrange a progression in or out of its normal sequence. While playing, it is important to have the functions in mind : tonic, dominant, subdominant, double- and secondary dominant.
A third chord progression, here called circular motion, can often be found as the middle part of a prelude. These different versions of the circular motion are taken from Cours de Composition Musicale written by the famous composition teacher Anton Reicha (1770-1836).