The Clavier-Übung III, sometimes referred to as the German Organ Mass, is a collection of compositions for organ by Johann Sebastian Bach, started in 1735–36 and published in 1739. It is considered Bach's most significant and extensive work for organ, containing some of his musically most complex and technically most demanding compositions for that instrument.

In its use of modal forms, motet-style and canons, it looks back to the religious music of masters of the stile antico, such as Frescobaldi, Palestrina, Lotti and Caldara. At the same time, Bach was forward-looking, incorporating and distilling modern baroque musical forms, such as the French-style chorale.

The work has the form of an Organ Mass: between its opening and closing movements—the prelude and "St Anne" fugue in E-flat, BWV 552—are 21 chorale preludes, BWV 669–689, setting parts of the Lutheran mass and catechisms, followed by four duets, BWV 802–805. The chorale preludes range from compositions for single keyboard to a six-part fugal prelude with two parts in the pedal.

The purpose of the collection was fourfold: an idealized organ programme, taking as its starting point the organ recitals given by Bach himself in Leipzig; a practical translation of Lutheran doctrine into musical terms for devotional use in the church or the home; a compendium of organ music in all possible styles and idioms, both ancient and modern, and properly internationalised; and as a didactic work presenting examples of all possible forms of contrapuntal composition, going far beyond previous treatises on musical theory.

The last manualiter four-part chorale prelude, Jesus Christus, unser Heiland BWV 689 in C minor, is marked "Fuga super Jesus Christus, unser Heyland" in the 1739 print. In contrast to the previous fughettas in the previous five manualiter settings of the catechism hymns, it is a long and complex fugue of great originality, a tour de force in the use of stretti. The fugue subject is derived from the first line of the chorale. In order to facilitate the stretti which underlie the whole conception of BWV 689, Bach chose to transform the modal melody by sharpening the fourth note from a B-flat to a B-natural, a modification already found in 17th-century hymnbooks. This change also allowed Bach to introduce dissonances, imbuing the work with what the French organist and musicologist Norbert Dufourcq called "tormented chromaticism".[57] The quaver countersubject and its inversions are used and developed throughout the fugue. It resembles some of Bach's other keyboard fugues, in particular the ante-penultimate fugue in B-flat minor BWV 891/2 of the second book of the Well-Tempered Clavier, composed at roughly the same time.

Source: Wikipedia (https://en.wikipedia.org/wiki/Clavier-%C3%9Cbung_III).

Although originally created for Organ, I created this Interpretation of the Chorale Prelude (BWV 689) "Jesus Christus, unser Heiland" (Jesus Christ our Saviour) for Flute & String Trio (Violin, Viola & Cello).