This fugue is standard repertoire for guitarists. A closer look shows that Bach uses a series of sequences for his sound in this piece, opting for a dozen scales to create the changes that are as follows: Major, Melodic, Harmonic Major, Harmonic minor, Neapolitan minor, Hungarian minor, Harmonic Major #4, Neapolitan Major, Locrian natural 7, Persian, Neapolitan Major b5 and Harmonic Major b5 and occur in that order.
There are a number of interesting sequences, chains of modulations that lead from one key to another or back to the original key. The first from bars 28 - 31:
C Major - D Melodic - D Major - A Major - A Melodic - G Major - B Neapolitan minor - E Hungarian minor - E Harmonic minor.
You can see the idea in this sequence which begins with a series of (+1) alterations:
NB: (+1) alterations are when the root of a mode is raised, conversely, (-1) are when the root of a mode is flattened.
1) C Major – C Ionian (+1) = C# Altered (D Melodic)
2) D Melodic – F Lydian + (+1) = F# Phrygian (D Major)
3) D Major – G Lydian (+1) = G# Locrian (A Major)
4) A Major – C# Phrygian (-1) = C Lydian + (A Melodic)
5) A Melodic – G# Altered (-1) = G Ionian
6) G Major – A Dorian (+1) = A# Alt bb3 (B Neapolitan minor)
7) B Neapolitan minor – D Dominant + (+1) = D# Alt nat5 bb7 (E Hungarian minor)
E Hungarian minor – A# Locrian bb37 (-1) = A Dorian #4 (E Harmonic minor)
Notice the order of (+1) alterations compared to (-1) alterations, 3x (+1), 2x (-1), 2x (+1), 1x (-1). There is always a sense of symmetry or order in Bach’s sequences, in this case, a diminishing series of 3 – 2, 2 – 1. Bach’s music, like nearly all music from the baroque period, uses incremental changes, altering one note at a time to reach the desired destination. This later became two simultaneous changes in the classical period and culminates in the contemporary period with multiple changes occurring almost continuously, such as in Takemitsu’s music. But more on that at a later date.
The second sequence continues from bars 32 - 36:
E Harmonic minor - E Hungarian minor - E Harmonic Major #4 - B Neapolitan Major - A Melodic - A Major - A Melodic - G Major - G Melodic - F Major - E Locrian natural 7 - B Persian - B Locrian natural 7 - B Neapolitan Major b5 - A Harmonic minor - C Major - E harmonic minor.
This second sequence is unusual because of the change to the Locrian natural 7, which Bach uses very rarely. The changes that lead from E Harmonic minor back again, a circular sequence, are:
1) E Harmonic minor – A Dorian #4 (+1) = A# Locrian bb37
(E Hungarian minor)
2) E Hungarian minor – G Ionian +#2 (+1) = G# Aeolian b4 ( E Harmonic Major #4)
3) E Harmonic Major #4 – D# Phrygian bb7 (-1) = D Lydian 7+ (B Neapolitan Major)
4) B Neapolitan Major – A# Alt bb3 (-1) = A Melodic (mode I of A Melodic)
5) A Melodic – C Lydian + (+1) = C# Phrygian (A Major)
6) A Major – C# Phrygian (-1) = C Lydian +
7) A Melodic – G# Altered (-1) = G Ionian (G Major)
G Major – B Phrygian (-1) = Bb Lydian + (G Melodic)
9) G Melodic – F# Altered (-1) = F Ionian ( F Major)
10) F Major – D Aeolian (+1) = D# Alt bb367 (E Locrian nat7)
11) E Loc nat 7 – Bb Lydian #3 (+1) = E Neapolitan minor AND A Aeolian #4 (+1) = A# Locrian bb367 (B Persian)
12) B Persian – D# Alt nat 5bb37 (-1) = D Dorian + (B Locrian nat7)
13) B Loc nat 7 – G Dominant #2 (+1) = G# Alt nat2bb7 (B Neapoiltan Major b5)
14) B Neapolitan Major b5 – A# Alt bb36 (-1) = A Harmonic minor (mode I)
15) A Harmonic minor – G# ALtbb7 (-1) = G Mixolydian ( C Major)
16) C Major – simultaneous raising of both F to F# AND D to D# (E Harmonic minor)
You can see in this sequence, Bach uses two simultaneous changes, the alteration of two notes at the same time. This is usually done via a chord or two altered notes that are following each other in quick succession, like 16ths. Bear in mind, one can have this number of alterations of bars or just beats, quite often Bach will change 7 or 8 times in one bar of music. That is when the sequence comes in useful.
The third sequence runs from bars 49 - 54:
D Harmonic minor - D Melodic - A Harmonic Major - A Harmonic Major b5 - A Persian - D Neapolitan minor - G Hungarian minor - D Harmonic Major - D Harmonic minor.
Without noting all of the changes, you can see this is a circular sequence once more, D Harmonic minor being the start and end point. Now you can understand the bigger picture, the previous circular sequence was on E Harmonic minor, this one is now in D Harmonic minor but Bach chooses to use different alterations to make it stand out from the previous one by using scales that are new, namely Harmonic Major and Harmonic Major b5. What this actually means is that the notes that are altered in any key are different in order to make it sound distinct from other parts of the music but also refreshing.
Think about it, in C Major you would alter F to F# to get to G Major, B to Bb for F Major, C to C# to D Melodic and G to G# for A Harmonic. You have to start changing the other notes, like A to A# or D to Db in order for it not to sound repetitive or predictable. Hence the more unusual scales. The standout modulation is the Harmonic Major b5 to Persian sequence, a very mature Bach sound which he uses often in his later music.
The fugue ends with a bang from bars 93 - 96:
A Harmonic minor - A Hungarian minor - A Harmonic minor - A Harmonic Major - A Harmonic minor - A Neapolitan minor - G Melodic - D Harmonic Major -D Harmonic minor - D Hungarian minor - A Harmonic Major - A Hungarian minor - E Harmonic Major.
Look closely at this sequence, for that is what it is. Not a series of accidentals appearing at certain places in the music, but a sequence that Bach has designed. In A Harmonic minor, raise and then drop the 4th, raise and then drop the 3rd then drop the 2nd to reach A Neapolitan minor. Now we rise through the circle of fifths using the Harmonic Major in the rest of the diminishing sequence: D Harmonic Major followed by two changes in D, A Harmonic Major followed by one change in A, and E Harmonic Major to finish. The pattern is H Major – 2 changes, H Major – 1 change, H Major.
Any of these changes or sequences can be borrowed and used in any other music, whatever the style and it still be unique and original as only the series of alterations are adhered to, not the melody, harmony or counterpoint. I think Bach had a musical idea and could apply many complex sequencies that had become like second nature to him, which like anything in music, only takes practise.
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The Modal Method explains all of this in more depth: https://www.bedwellmusic.co.uk/general-7
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