The first book of J. S. Bach's Das Wohltemperierte Klavier (The Well-Tempered Clavier) was complete by 1722. Bach gave the present title to the work, which was composed "for the use and practice of musical youth eager to learn and for the amusement of those already skilled in this study." Bach composed a paired Prelude and Fugue in each of the 24 keys to demonstrate the viability of the new "equal-temperament" system, which allows one to play in all keys without producing out-of-tune intervals, as happened with Pythagorean and "mean tone" tunings. Furthermore, the pieces are as much compositional studies as keyboard works.

Twenty years later, Bach assembled another set of preludes and fugues. The title page is missing from the manuscript, but its similarity to the "first" book of The Well-Tempered Clavier led editors to entitle it "Book II." Bach worked on the second book over a long period of time, even reworking pieces he had written for other purposes, as he had in the first book. Some of the preludes and fugues date from the 1720s. Possibly the most substantial revision for the second book was to No. 3, in C sharp major, which was originally in C major. Fugues Nos. 15 and 17 survive in earlier versions in which they are connected with different preludes than we find in The Well-Tempered Clavier.

There are many musical differences between the works of Book II and those of Book I. The preludes in the second group explore a greater range of forms and styles than do the earlier examples. Most striking are the experiments in the style of the Italian bipartite keyboard sonata, codified by Domenico Scarlatti. Like earlier dance movements, these consist of two repeated sections. Some of the "sonata" preludes in Book II, such as Nos. 5, 12, and 21, feature a recapitulation of the opening material, while others (Nos. 10 and 15) have parallel closing sections. Prelude No. 17 is an Italian concert-ritornello movement; Nos. 13 and 23 also display concerto traits. The tenth is a two-part invention and Nos. 4 and 14 are ariosos. Those that are clearly derived form dance forms -- No. 5 from the gigue and No. 8 from the allemande -- lack the traditional binary form.

This prelude takes the form of a substantial courante, initially with a three-voice texture. But Bach frequently abandons one of the voices to accommodate the hand crossings that pervade this piece. Next comes a three-voice fugue, starting in the alto and then entering in the soprano and bass. This little fugue has elements of the trio sonata minuet, and is based on a nervous subject of stuttering eighth notes. Bach manages to pack in two further thematic elements, both fragments of an ascending scale; they combine with the main subject upon their arrival and again toward the fugue's end.

Source: Allmusic (http://www.allmusic.com/composition/prelude-and-fugue- for-keyboard-no-21-in-b-flat-major-wtc-ii-21-bwv-890-bc -l124-mc0002391745).

Although originally written for Harpsichord. I created this Arrangement of the Prelude & Fugue in Bb Major (BWV 890) for String Trio (Violin, Viola & Cello).