Mass, for double choir and 2 organs, op. 36

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 most musically complex and technically 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♭ major, BWV 552—are 21 chorale preludes, BWV 669–689, setting two parts of the Lutheran Mass and six catechism chorales, 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.