(con't from Part XXXVI)
"Start by doing what's necessary; then do what's possible; and suddenly you are doing the impossible."
-- Saint Francis of Assisi
The note upon which the 1st voice of an organ fugue starts is subject (no pun intended) to certain general principles with which the would-be fugue writer needs to be thoroughly familiar.
It's unrealistic to expect any online crash course in fugue writing to turn an organist into a fugue writer overnight, but there is much, very much, about it that can be taught online provided full and correct information is provided and the learner is stubbornly determined to understand it.
The following will illustrate this:
The 2nd voice to enter typically repeats the 1st voice, or subject, transposed up a 5th or down a 4th in the dominant tonality; this is called the answer.
The dominant key is typically chosen for the answer because scale degree 5 is the 3rd harmonic (2nd overtone) of the natural harmonic series and, save for the tonic and its octave, is the next most important note of the scale; when the answer begins in the dominant tonality it sets up a tension with the subject which has just entered in the tonic; this tonic-dominant tension is what drives all tonal music in the Western world.
The 1st note of the answer, however, and sometimes a few others as well, is sometimes transposed a 4th higher; the reason for this discrepancy has to do with harmony.
If the answer is to enter as the subject finishes, as it usually does, it generally needs to begin on a note that's consonant with tonic harmony or it will enter jarringly on an unprepared dissonance (although these days, dissonance is the norm).
For a subject starting on scale degree 1, the answer can begin a 5th higher, on 5, which is consonant with the tonic harmony implied when the answer begins at the end of the subject, but if the subject begins on 5, the answer cannot start a 5th higher, on 2, because 2 is dissonant with tonic harmony.
Consequently, the first note or two of the answer is adjusted to begin on 1, a 4th higher, in order to begin on a consonance; this type of adjustment is called a tonal answer because it has been modified from its strict 5th transposition, this in order to confirm the tonality (tonic) by entering on a consonance, rather than conflict with it by entering on a dissonance.
After this adjustment the remaining notes in the answer are all 5th transpositions of the subject.
We read in various sources that the appearance of scale degree 5 at or near the beginning of the subject is the signal that that answer MUST be tonally adjusted, whereas if 5 does not appear at or near the beginning of the subject, no adjustment is necessary, and we speak of a real answer (a literal transposition of the subject, note for note).
The fact is, in rare cases, it's entirely possible for the answer to appear in the subdominant, a 4th higher than the subject rather than a 5th, with the result that no tonal adjustment in the answer may be needed and a real answer might be supplied.
In this case the subject starting on the 5th scale degree is sufficient to set up a tonic-dominant tension; in such a situation, after the bridge passage following the answer brings about a return to the tonic key for the 3rd entry, the 4th entry will also be in the subdominant tonality, i.e., a 4th higher.
Due to this alternative, those who maintain that all fugues which begin on scale degree 5 MUST have a tonal answer are ignoring exceptions for the time being -- most likely to steer the novice away from what they consider to be any serious entanglements in their first attempts at fugue writing.
One might only examine the expositions in Bach's great d minor Toccata & Fugue for organ BWV 565 or this author's b minor Prelude & Fugue for organ Op. 26 for proof that this can be done, and there are other examples.
Students not fully informed by this could begin to surmise, using the same reasoning, that fugue subjects DO NOT start on scale degree 3; they can presume that the answer, in this case, would have to begin on 7 (leading tone), a 5th higher than 3, an impossibility considering that 7 is built upon a diminished triad.
When the interested student is directed to "the 48" and asked to find a single fugue there which starts on scale degree 3 and they fail to find one, they're tempted to accept this as proof that such a proceeding isn't feasible.
Except that this isn't where it is: Kirnberger is on record for stating that his master JSB who composed of "the 48" believed that everything in composition was possible and would never hear of anything being not feasible.
In the case of a subject which has a tail which points strongly to the dominant key it's entirely possible for that subject to start on scale degree 3 provided that it's supplied with a tonal answer.
From this a corollary might be drawn to preserve consonant tonal harmony: any fugue subject starting on the mediant note needs to modulate to the dominant and be supplied with a tonal answer.
If often happens in education that those individuals upon whom we habitually rely for trustworthy information can give us all the basics about it that we can stand; the trouble is, most students who want to really penetrate to the core of a study need more than the basics, and they need it all the time.
As a fugue writer J.S. Bach was coached at an early age by his older brother who had been a pupil of Pachelbel and who studied Pachelbel's organ fugues carefully; young JSB was mostly self-taught however and penetrated to the core of his art by means of sheer hard work, an obsession to excel, a dogged determination never to give up, and by doing whatever had to be done to prepare for his work-a-day existence as a practical musician.
The trail he took as an emerging composer led him down every blind alley as his mastery developed; this school-of-hard-knocks is an arduous path full of cruel uncertainties; it's also the path that makes for a really good teacher.