Bach’s method of teaching composition was equally sure and effective.(178) He did not begin with the dry details of counterpoint, as was the custom of other teachers in his day. Still less did he burden his pupils with the physical properties of sound, which he held to be matter for the theorist and instrument-maker rather than the composer. He started them off at once on four-part harmony over a figured Bass, making his pupils write each part on a separate stave in order to impress on them the need for accurate harmonic progression. Then he passed to Hymn tunes, setting the Bass himself and making his pupils write the Tenor and Alto parts. In time he let them write the Bass also. He insisted on correct harmony and on each part having a real melodic line. Every musician knows what models Bach has left us in this form. The inner parts of his four-part Hymn-tunes are so smooth and melodious that often they might be taken for the melody. He made his pupils aim at similar tunefulness, and until they showed a high standard of merit did not permit them to write compositions of their own. Meanwhile he aimed at cultivating their feeling for pure harmony and for the order and connection of ideas and parts by familiarising them with the compositions of others. Until they had acquired facility in those qualities he neither permitted them nor held them competent to put pen to paper.
Bach required his pupils in composition to work out their musical ideas mentally. If any of them lacked this faculty he admonished him not to compose and discountenanced even his sons from attempting to write until they had first given evidence of genuine musical gifts. Having completed their elementary study of harmony, Bach took his pupils on to the theory of Fugue, beginning with two-part writing. In these and other exercises he insisted on the pupil composing away from the Clavier.(179) Those who did otherwise he ridiculed as “Harpsichord Knights.” In the second place he required rigorous attention to each part and its relation to the concurrent parts, permitting none, not even an inner one, to break off before it had finished what it had to say. He insisted upon a correct relation between each note and its predecessor. If he came upon one whose derivation or destination was not perfectly clear he struck it out as faulty. It is, indeed, a meticulous exactitude in each individual part that makes Bach’s harmony really multiple melody. Confused part-writing, where a note that belongs to the Tenor is given to the Alto, or vice versa, or the haphazard addition of extraneous parts to a chord which suddenly shows an increase of notes as if fallen from the sky, to vanish as suddenly as they came, are faults found neither in his own nor his pupils’ writing. He regarded his musical parts as so many persons engaged in conversation. If there are three, each of them on occasion may be silent and listen to the others until it finds something relevant to say itself. But if, at an interesting point of the conversation, an interloping voice intervened, Bach regarded it as an intruder and let his pupils understand that it could not be admitted.