harmonizations: modulations
One of the most important principles of a prelude is that it ends in the same key as it started. This does not prevent the pianist from performing modulations within the prelude itself. However, there is also another type of prelude that has a specific function, called a modulating prelude or simply modulation. This type of prelude is used to make a connection between two pieces of repertoire with different keys.
In his publication The Art of Preluding, Op. 300 (1833), Carl Czerny (1791-1857) provides various modulating chord progressions:
Carl Gottlieb Hering (1766-1853) provides almost a thousand 'Ausweichungen' in his Praktische Preludierschule (ca.1812).
MODULATION SYSTEMS
Rather than relying on ready-to-use chord progressions to modulate to a new key, one can benefit from understanding the 'systems' of modulation. In his publication Modulations (ca.1888), Bernhard Rollfuss (1837-1904) gives a clear view on some of the possible modulation systems, for example modulation by a diminished chord:
Some more interesting publications regarding modulations:
Bernhard Rollfuss (1837-1904)
Modulations (ca.1888)
Salomon Jadassohn (1831-1902)