Bach mapped all twenty-four keys so every door would open in tune
The Well-Tempered Clavier, BWV 846–893, is Johann Sebastian Bach's two books of prelude-and-fugue pairs in all twenty-four major and minor keys. Book 1's 1722 title page announced a well-tempered clavier—keyboard tuning that keeps every key usable—helping make the set a cornerstone of Western keyboard literature.
Each book climbs chromatically from C major and C minor through every tonal center until B minor—twenty-four pairs per volume. Bach reused earlier material; the 1720 Klavierbüchlein for Wilhelm Friedemann holds versions of eleven Book 1 preludes. J. C. F. Fischer's Ariadne musica neo-organoedum (published 1702 and reissued 1715) offered twenty prelude-fugue pairs in related keys plus chorale-based pieces and likely influenced Bach. Earlier cycles by Giacomo Gorzanis, Vincenzo Galilei, and others had already toyed with walking through many keys before modern tonality settled.
Well temperament is not automatically equal temperament. Mean-tone systems left remote keys harsh; Bach's title implies a circulating tuning where all keys work, and Forkel reported Bach tuning his own harpsichords and clavichords to play everywhere with varied key colors. Twentieth-century players often presumed equal temperament, possibly mistakenly. Since 1950 performers have tried many unequal schemes; John Barnes's statistical study of major-key preludes and Herbert Kellner's numerological proposals are among contested modern guides.
A doodle of loops on Bach's 1722 title page has inspired rival decoding by Andreas Sparschuh, Michael Zapf, Bradley Lehman, and others who read fifth sequences or beat rates into the swirls. Other musicologists, including David Schulenberg, say the drawing may mean nothing reliable—students did not copy the loops carefully—and Bach may have tuned differently by occasion. Whatever the temperament, the pedagogical and artistic project—fully worked pieces in every key—remains unmatched in ambition for its century.
Source: The Well-Tempered Clavier
