If we discount the D minor Toccata and Fugue, BWV 565, made famous by the motion picture Fantasia, J. S. Bach's best-known organ work is probably the chorale prelude Kommst du nun, Jesu, vom Himmel herunter, BWV 650. It is a magnificent example of the pedal cantus firmus style. Not nearly so dense and motivically organized as the preludes in the Orgelbüchlein, it is instead an expansive work in which a delightfully flowing, newly composed obbligato melody is every bit as vital as the Lutheran chorale that serves as its cantus firmus backbone.
The cantus firmus of BWV 650 is a relatively late Lutheran hymn (1665) that Bach used on several occasions and with several different texts. In fact, the organ prelude was transcribed directly from the second movement of the cantata Lobe den Herren, BWV 137 of 1725 (a movement in which Bach sets the "Lobe den Herren" text to the melody), a little over two decades after the cantata was originally composed. The chorale prelude version of the music is called Kommst du nun, Jesu, vom Himmel herunter solely because that is the title given to the piece in the 1749 volume (the so-called Schübler Chorales -- all transcriptions of earlier cantata movements) in which it first appeared; precisely what the connection in Bach's mind between the melody and the Nativity text "Kommst du nun" was is not entirely certain, though the text and melody certainly fit together seamlessly.
The actual cantus firmus melody -- played on the pedals, but using a stop the ensures that the resulting pitch is higher than that of the manual left hand (which is thus the true bass of the piece) -- only appears in isolated pockets of the prelude, each time rising up out of the flowing right hand's obbligato melody as if to bridge the distance between Heaven and earth ("Kommst du nun, Jesu, vom Himmel herunter auf Erden"; "Are you coming now, Jesus, from Heaven down to earth"). The obbligato line is itself built of such persuasive stuff that it continues for a full twelve measures after the cantus firmus has had its final say, finally winding down only at the final, rich cadence. ~ All Music Guide
