The chorale harmonization is a way to learn about nonharmonic tones. The works of J.-S. Bach serve as supreme models in the application of such nonharmonic tones. With respect to the problem of matching voices to given melodies, nowhere is this more true than in the 371 chorale harmonizations, four-part settings of hymn tunes from the Protestant rites (Luther). The student will find profitable to examine these chorales frequently, especially during the first years of harmony study. A number of chorales appear in several harmonizations, which are well worth comparing. Many of them contain at least one modulation, those in the minor mode often haven diminished seventh chords, and nearly all the chorales make use of secondary dominants and non dominant sevenths. It is instructive for the student to try his hand at chorale harmonization: two or three different versions should be made for each phrase, including one using only triads in root position and first inversion without nonharmonic tones. The chorale must be given as a melody ready for a four-part vocal harmonization. The variation has been composed for Organ with respect to the model "Ich ruf zu dir, Herr Jesu Christ BWV 639 (in J.-S. Bach's Orgelbüchlein), using the two obliged - continuous - lines of eights (left hand) and fourth notes (Pedals).