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History
Early versions of almost all the chorale preludes are thought to date back to 1710–1714, when Bach was court organist and concertmaster in Weimar, at the court of Johann Ernst III, Duke of Saxe-Weimar. When he moved to his later positions as kapellmeister in Cöthen in 1717 and cantor at the Thomaskirche in Leipzig in 1723, his obligations did not specifically include compostions for the organ. The autograph manuscript of the Great Eighteen, currently preserved as P 271 in the Berlin State Library, documents that Bach began to prepare the collection around 1740, after having completed Part III of the Clavier-Übung in 1739. The manuscript is made up of three parts: the six trio sonatas for organ BWV 525–530 (1727-1732); the "Canonic Variations" on Von Himmel hoch BWV 769 added at the same time as the chorale preludes (1739-1750); and an early version of Nun komm' der heiden Heiland (1714-1717), appended after Bach's death. The first thirteen chorale preludes BWV 651–663 were added by Bach himself between 1739 and 1742, supplemented by BWV 664 and 665 in 1746–7. After 1750, Bach began to suffer from blindness until his death in 1751, BWV 666 and 667 were dictated to his student and son-in-law Johann Christoph Altnikol. Only the first page of the last choral prelude BWV 668, the so-called "deathbed chorale", has survived, recorded by an unknown copyist. The piece was posthumously published in 1751 as an appendix to the Art of the Fugue, with the title Wenn wir in höchsten Nöthen sein (BWV 668a), instead of the original title Vor deinen Thron tret ich hiermit ("Before your throne I now appear"). There have been various accounts of the circumstances surrounding the composition of this chorale. The biographical account from 1üü802 of Johann Nicolaus Forkel that Atnikol was copying the work at the composer's deathbed has since been discounted: in the second half of the eighteenth century, it had become an apocryphal legend, encouraged by Bach's heirs, Carl Philip Emmanuel Bach and Wilhelm Friedmann Bach. The piece, however, is now accepted as a planned reworking of Wenn wir in höchsten Nöthen sein (BWV 641) from the Orgelbüchlein (c 1715).

Quotations

 * According to Manfred Bukofzer, the Great Eighteen "transcend by their magnitude and depth all previous types of choral prelude."
 * The Bach scholar Albert Schweitzer, although hailing the Great Eighteen as "masterpieces", preferred the shorter and more compact chorale preludes of the Orgelbüchlein, where the accompaniment followed the cantus fermus, rather than providing an independent voice filling the long intervals between the successive lines of the cantus fermus.
 * Harvey Grace, commenting on the difference between the miniature intimacy of the Orgelbuchlein and the epic grandeur of the Great Eighteen, remarked that they display a "workmanship as nearly flawless as we have any right to expect of a human being."

Compositional models
The breadth of styles and forms represented by the Great Eighteen is as diverse as that of Bach's Well Tempered Clavier for the keyboard. The pieces are on a large and often epic scale, compared with the miniature compactness of the choral preludes of the Orgelbüchlein. Many of the chorale preludes pay hommage to much older models in the German liturgical tradition (Böhm, Buxtehude and Pachelbel), but the parallel influence of the Italian concerto tradition is equally visible. It is a mid-eighteenth century salute to the musical traditions of the previous century. Unlike Part III of the Clavier-Übung, where Bach pushed his compositional techniques for the organ to new limits, the chorale settings of Bach's Great Eighteen represent "the very quintessence of all he elaborated in Weimar in this field of art;" they "transcend by their magnitude and depth all previous types of choral prelude"; and they display a "workmanship as nearly flawless as we have any right to expect of a human being." The eighteen are characterized by their freely developed and independent accompaniment filling the long intervals between the successive lines of the cantus fermus, a feature of their large scale which has not pleased all commentators.

Chorale motet
The Rennaissance motet, in madrigal style, forms the model for the chorale motet, used in BWV 652, 665 and 666. Each line of the chorale is established as a point of imitation for the different parts, which keep to a common rhythm. This style, the earliest used by Bach, was that employed in his Mühlhausen cantatas, such as the funeral cantata Actus Tragicus, BWV 106.

Chorale partita
The chorale partita is a set of variations on chorale melody. Normally each variation repeats the chorale melody and is essentially a separate movement. This style goes back to the Dutch composer Sweelinck and was adopted by his German pupils Scheidt and Scheidemann; the tradition was continued at the turn of the 18th century by Böhm and Pachelbel from Thuringia, who provided the model for Bach. Bach, however, broke the norm in the two chorale preludes of this genre, BWV 656 and 667, which each have only a small number of variations (3 and 2). This might be a hommage to Buxtehude, who had written similar partitas and whose music and virtuosity at the organ is known to have exercised a considerable influence on Bach in his youth.

Ornamental chorale
In the ornamental chorale, a form invented and popularized in Northern Germany by Scheidemann, the chorale melody is taken by one voice in an elaborate and highly embellished form. Buxtehude was one its most celebrated exponents, with his individual expressive "vocal" ornamentation. Five chorale preludes of the Great Eighteen were written in this style: BWV 652, 653, 654 and 662.

Cantus firmus chorale
The cantus firmus chorale. the melody of the chorale is sounded in long notes throughout the piece, was established and popularized in central Germany by Pachelbel. One of his students was Johann Christian Bach, Bach's older brother, who in turn taught Bach keyboard technique. There are five examples of the cantus firmus chorale: BWV 651, 657, 658, 661 and 668.

Chorale trio
The chorale trio has the form of a trio sonata in which the upper parts are played on the two keyboards of the organ and the basso continuo part is played on the pedals. Bach elevated this form to the status of the contemporary Italian trio sonatas or double concertos: it is probably his single most original and innovative form of the organ chorale. The two virtuosic chorale preludes of this type are BWV 663 and 664.