The collections of Preludes and Fugues in all keys, major and minor, known as The Well-tempered Clavier, or, from their number, as The Forty-Eight, explore the possibilities inherent in every possible key. Experiments in keyboard tuning in the later seventeenth century had resulted in differing systems that, nevertheless, made the use of remoter keys feasible. While the precise nature of the tuning system used by Bach may not be clear, his well-tempered tuning at least made all keys possible, although, in the system of equal temperament employed, some keys were probably more equal than others, an effect lost in modern piano tuning.

The second book of Preludes and Fugues in all twenty-four keys, twelve major and twelve minor, was assembled for publication in 1742, drawing to some extent on compositions from Bach's period at Cöthen and, more largely, from the work of recent years. While the Preludes vary in mood and form, the Fugues are bound by stricter rules of counterpoint, in which a subject is announced, to be answered in imitation by a second, third and fourth voice. The answer may be accompanied by a countersubject, a secondary theme that fits with the subject, but, has its own characteristics. Intervening episodes appear between further entries of the subject in other keys from any of the voices or parts. Other devices include the use of stretto, the overlapping entry of voices with the subject. Further complementary subjects may appear, again entering in imitation by one voice of the other, and may be combined with the original subject. The subject itself may appear in inversion, upside down, or in augmentation, with longer notes, or diminution, with shorter and quicker note-values. True art is to conceal art, and this Bach, as always, achieves in music that is never subservient to technical requirements. The Preludes and Fugues were written for unspecified keyboard instrument, with some suggesting rather the gentle tones of the clavichord, others the louder harpsichord and some even the sustained notes of the organ.

The Prelude in C major opens over a sustained tonic pedal to impressive effect. The three-voice fugue has its opening subject in the alto voice, answered in the soprano, followed by a bass entry. It is followed by a more rapid C minor Prelude and a four-voice fugue, with entries in the order alto, soprano, tenor and bass. The overlapping final entries in the contrapuntal device of stretto lead to a solemn conclusion. Moving up a semitone to C sharp major, the third prelude is gently lyrical, leading to its own quicker miniature fugue, before the three-voice fugue proper, its entries, bass, soprano and alto overlapping in stretto, with the third entry inverted.

The C sharp minor Prelude, with its three voices interweaving, is capped by a gigue like fugue, its entries in the order bass, soprano and alto, with a slower subject appearing in the middle of the fugue and serving as a countersubject. The Prelude in D major is marked by asymmetry in rhythm. The four-voice fugue has subject entries in the order tenor, alto, soprano and bass, the third and fourth entries overlapping in stretto. The following D minor Prelude is a brilliant two-voice composition. Its three-voice fugue, with entries alto, soprano and bass, offers contrasting rhythms.

The use of the appoggiatura adds to the lyrical nature of the Prelude in E flat major, coupled with an alla breve fugue in four voices, entering in ascending order. The enharmonic D sharp minor, with six sharps, is used for the next prelude and fugue, the first in two-voice texture in the manner of a two-part invention, leading to a four-voice fugue, with voices entering in the order alto, tenor, bass and soprano and a countersubject of initial importance accompanying the second entry. The subtly sustained notes of the opening of the E major Prelude provide a clear harmonic pattern. The alla breve four-voice fugue, with entries in ascending order, makes considerable use of stretto.

The E minor Prelude is a two-part invention. Its three-voice fugue, with entries in descending order, has a subject of contrasted rhythms and ends in imposing style. The F major Prelude is of some complexity, as voice is added to voice in a five-part texture. To this the three-voice fugue, with entries in descending order, provides a lighter contrast. There is an almost rhetorical air about the F minor Prelude, coupled with a lively three-voice fugue, with voices entering in descending order.

Prelude No. 13 in F sharp major opens with an upper voice melody, using rhythmic figures that re-appear throughout the movement. The dance-like three-voice fugue, with entries in the order alto, soprano, bass, with an important countersubject accompanying the second and third entries. The key of F sharp minor allows a solemn prelude with an upper part melody of rhythmic variety. The three-voice fugue, with entries in the order tenor, soprano and bass, has further subject material in two other places, as the fugue develops, these three subjects later combined.

The Prelude in G major is of simpler texture, with a slighter three-voice fuguetta in which voices enter in descending order with the arpeggios of the subject. This is followed by a G minor Prelude, marked Largo by the composer and using the dotted rhythms of the opening of a French overture, followed by a majestic four-voice fugue, with voices entering in the order tenor, alto, soprano, bass, and a strongly characterized countersubject, the whole leading to an imposing climax. The A flat major Prelude allows emphasis on the tonic chord and the subdominant in its opening bars, using figuration that has a later part to play in the texture. The four fugal voices enter in the order alto, soprano, tenor and bass, its countersubject a series of descending chromatic notes. The following prelude uses the enharmonic key of G sharp minor and includes, unusually, contrasting dynamic markings, suggesting something of the dramatic rhetoric of the new age. The companion three-voice fugue has its entries in descending order. There is a later chromatically descending subject, introduced in all three voices and later combined with the original subject.

The Prelude in A major is a gentle three-voice piece in which the 12/8 metre suggests a pastoral mood. The three-voice fugue, with entries in ascending order, contrasts the rhythm of the subject with an accompanying dotted rhythm. The A minor Prelude is chromatic in its lyrical two-voice texture and is paired with a three-voice fugue with entries in ascending order with a short and wide-spaced subject broken by rests. The rapider notes of the countersubject assume importance, as the fugue proceeds. The following B flat major Prelude starts with a three-voice texture that is at times abandoned, particularly with the crossing of parts and hands that is a feature of the writing. The alto announces the subject of the three-voice fugue, followed by soprano and bass; the subject itself in a quaver rhythm with a suggestion of appoggiatura in its second half. Two other thematic elements appear, based in both cases on the ascending scale and these combine with the subject at their first appearance and in the conclusion of the fugue. The B flat minor Prelude starts with a melody in the middle part of a three-voice texture, aided by the entry of the third, upper part. The alto states the extended fugal subject, answered by soprano, followed by bass and then tenor in a four-voice texture. The later entries are accompanied by a strongly characterized ascending chromatic countersubject. The subject lends itself to the use of overlapping entries in stretto, either closely juxtaposed or more widely spaced apart.

The B Major Prelude, in a form that suggests a toccata, is coupled with a four-voice fugue, in which the voices enter in ascending order. There is a countersubject of marked contrast, accompanying each entry. The book ends with a B minor Prelude and Fugue. The first of these, in two-voice texture, has the structure of a two-part invention, the lower part providing an imitation of the upper in a repetition of the subject at the octave, with later entries in related keys. The alto is entrusted with the first statement of the fugal subject, followed by soprano and then bass. The countersubject includes a passing imitation of the subject itself, which appears in stretto. A second subject is introduced by the bass in accompaniment to the second entry of the subject, and is thereafter used as an accompaniment to the subject. The B minor Fugue brings to an end a remarkable work that set, for all time, an example of contrapuntal keyboard writing, in all its possible variety, in which technical devices are deployed with absolute mastery.

Writer: Keith Anderson