Johann Sebastian Bach finished the first part of Das Wohltemperierte Klavier in 1722 and the second in 1740/42. The title page promises preludes and fugues durch alle Tone und Semitonia, through all tones and semitones, zum Nutzen und Gebrauch der Lehrbegierigen Musicalischen Jugend, for the use of young musicians eager to learn.
Each part has 24 preludes and fugues, one pair for every major and minor key. In notes from his studies with the conductor Hans von Bülow, Theodor Pfeiffer records the sentence Das wohltemperirte Clavier ist das alte Testament, die Beethoven’schen Sonaten das neue, an beide müssen wir glauben. The Well-Tempered Clavier is the Old Testament, Beethoven’s sonatas the New, and we must believe in both.