Bach's The Well-Tempered Clavier organizes preludes and fugues by key, with each book covering twelve major and twelve minor keys. The title relates to keyboard tuning practices and should not simply be assumed to mean today's twelve-tone equal temperament. The work serves as practice material, a musical exploration, and an artistic achievement.
Historical background
The Well-Tempered Clavier, BWV 846–893, consists of two sets of preludes and fugues in all 24 major and minor keys for keyboard by Johann Sebastian Bach. In the composer's time clavier referred to a variety of keyboard instruments, namely the harpsichord, the clavichord and the organ (which operates using air instead of strings), but not the regal and the then newly-invented pianoforte.
The modern German spelling for the collection is Das wohltemperierte Klavier. Bach gave the title Das Wohltemperirte Clavier to a book of preludes and fugues in all 24 keys, major and minor, dated 1722, composed "for the profit and use of musical youth desirous of learning, and especially for the pastime of those already skilled in this study". 20 years later, Bach compiled a second book of the same kind (24 pairs of preludes and fugues), which became known as The Well-Tempered Clavier, Part Two.
Composition history
Each set contains 24 pairs of prelude and fugue. The first pair is in C major, the second in C minor, the third in C♯ major, the fourth in C♯ minor, and so on. The rising chromatic pattern continues until every key has been represented, finishing with a B minor fugue. The first set was compiled in 1722 during Bach's appointment in Köthen, and the second followed 20 years later in 1742 while he was in Leipzig.
Bach recycled some of the preludes and fugues from earlier sources: the 1720 Klavierbüchlein für Wilhelm Friedemann Bach, for instance, contains versions of eleven of the preludes of the first book of the Well-Tempered Clavier.
In Bach's own time just one similar collection was published, by Johann Christian Schickhardt (1681–1762), whose Op. 30 L'alphabet de la musique (circa 1735) contained 24 sonatas in all keys for flute or violin and basso continuo, and included a transposition scheme for alto recorder.
Intended tuning
Bach's title suggests that he had written for a 12 note tuning system, in which all keys sounded in tune (called a "circulating temperament" or a "well temperament"). One of the opposing systems in Bach's day was meantone temperament in which keys with many accidentals sound out of tune on keyboards limited to 12 pitches per octave. Bach would have been familiar with different tuning systems, and in particular as an organist would have played instruments tuned to a meantone system.
During much of the 20th century it was presumed, possibly mistakenly, that Bach intended equal temperament, which after Bach's death became popular as the standard keyboard tuning, and had been described by theorists and musicians for at least a century before Bach's birth. Accounts of Bach's own tuning practice are few and inexact. The three most cited sources are Forkel, Bach's first biographer; Friedrich Wilhelm Marpurg, who received information from Bach's sons and pupils; and Johann Kirnberger, one of those pupils.
Despite the presumption of equal temperament, research has continued into various unequal systems contemporary with Bach's career; there is debate whether Bach might have meant a range of similar temperaments, perhaps altered slightly in practice from piece to piece, or possibly some single, specific, "well-tempered" solution for all purposes. Modern scholars suggest some form of unequal well temperament instead of equal temperament.
Forkel reports that Bach tuned his own harpsichords and clavichords and found other people's tunings unsatisfactory, and also that Bach's personal tuning system allowed him to play in all keys, and to modulate into distant keys almost without the listeners noticing.
Style
Musically, the structural regularities of the Well-Tempered Clavier encompass a wide range of styles. The preludes are formally free, although many of them exhibit typical Baroque melodic forms, often coupled to an extended free coda (e.g. Book 1 preludes in C minor, D major, and B♭ major). The preludes are also notable for their odd or irregular numbers of measures, in terms of both the phrases and the total number of measures in a given prelude.