The "Schübler Chorales" is a name usually given to the "Sechs Chorale von verschiedener Art" ('Six Chorales of Various Kinds') for organ (BWV 645–650), a collection of six chorale preludes by J. S. Bach, issued around 1748. The title 'Schübler Chorales' derives from the engraver and publisher Johann Georg Schübler, who is named on the title page. All six of the preludes are for an organ with two manuals and pedal, at least five of them transcribed from movements in Bach's cantatas, mostly chorale cantatas.
The fourth one of the set is the one presented here, and it comes from the 5th part of BWV 10, and is a duet between alto and tenor.
The fact that Bach had gone to the trouble and expense of securing the services of a master engraver to produce a collection of note-for-note transcriptions of this kind indicates that he did not regard the Schübler Chorales as a minor piece of hack-work, but as a significant public statement.
This setting is also significant as the chorale melody is actually the "Tonus Peregrinus."
In Gregorian chant the tonus peregrinus existed before the modal system was expanded beyond the eight mode. Later the ninth tone became associated with the ninth mode, or Aeolian mode, which, in a more modern understanding of harmony, can be equalled with a standard minor mode.
The traditional German Magnificat, sung on a German variant of the ninth tone or tonus peregrinus. It is an exceptional reciting tone in Gregorian chant: there it was most clearly associated with Psalm 113 (114). In Lutheranism, the tonus peregrinus is associated with the Magnificat (also usually sung in vespers): the traditional setting of Luther's German translation of the Magnificat ("Meine Seele erhebt den Herren") is a German variant of the tonus peregrinus.
The original title page of the "Schübler Chorales" is attached below, as well as two images of Bach that were not familiar to me, including one issued as a commemorative postage stamp.