Thus, we must once again be wary of the monumentalizing tendencies in analysis, radicalized in the notion of organic unity, of course, but also through an inevitable tendency in the rhetoric of analysis and its presentation or argument, intensified in the publication-oriented authenticist biases of the nineteenth and early to mid-twentieth centuries. (Not to mention the hardening of attitude about a particular reading that often results from classroom repetition.) Perhaps I am myself more than usually sensitive to this at the moment, having just recently finished an essay whose repertoire draws heavily on eighteenth-century Scottish fiddle tunes (link).
In any case, Urlinien and other abstract shapes for BWV 924 and 924a must be regarded cum grano salis. Not with respect to their basic legitimacy as readings—the piece, in either version, is so short that one really can hear some of these shapes—but with respect to subsequent claims that might be made. That is to say, the informal nature of the Clavier-Büchlein and the presence of BWV 924a undermine any conclusion that one's analysis demonstrates just how BWV 924 is another perfect, gleaming jewel in J. S. Bach's compositional crown, another example of German genius, or another instance of a musical genius manipulating the "tonal system." What we can say certainly is that the two versions are evidence of practice in performance, improvisation, and composition.
The collated block-chord reductions below are intended to show how young Friedemann might have developed his own composition out of his source. First, we assume that he learned to play BWV 924, probably as given in the score but also as its bass line, to which he supplied upper voices in the manner of the Neapolitan partimento pedagogy. From this point, he would be expected to use the musical materials to fashion original pieces, the best of which was written into the Clavier-Büchlein as BWV 924a.
It is worth asking if BWV 924 is an exercise in composition, what is the task? What is the student's assignment?
If it is the bass figure, as in the simpler partimento exercises, then this is a very odd one. The figure of the opening is the rising fifth, so C-G-D-A-E. Bach stops only when the next chord would be an undesirable diminished triad in root position (middle of bar 3 below). In the various documents available on Robert Gjerdingen's Monuments of Partimenti website (link), I found only one "rule" (sample progression) focused on a sequence of rising fifths (link), but no partimento compositions. The only composition that features rising fifths in its opening is the very last of 44 by Fedele Fenaroli (link) and that uses the Romanesca bass rather than a simple sequence of rising fifths. In this connection, it is interesting that Friedemann abandons his father's sequence almost immediately and converts the figure into the Romanesca bass--at (a)--but then breaks that after four notes to continue in A minor--at (b). Fenaroli has a rule for the Romanesca bass immediately preceding the one mentioned above: link.
BWV 924 clearly also seems to be a lesson in suspensions, beginning with the "easy" ones -- 4-3 over root position triads -- then proceeding to the dissonant 6/5 pairs, then to 9-8. The positions for all these are shown with asterisks (*). (The only common type missing is the 2-3 bass suspension, which, of course, is prominent in the WTC I, C Major Prelude, a version of which also appears in the Clavier-Büchlein.) Note that Bach Vater continues the suspension work over the extended cadence dominant--see ** below; these are 7-6 figures between the upper voices. Friedemann, on the other hand, abandons suspensions altogether and has some fun with marching triads and dramatic arpeggios in the minor key.
BWV 924, ending:
Here is another graphic to compare the two versions: the reduced upper line only, up to the first part of the dominant pedal. Note how the same materials are used in each section, until Friedemann turns to triads (section 3) and reverses the direction of the line (section 4).
