The importance of movable Do (C) singing in Solfege system helps identify incorrect notes many guitarists play on Bach's music.
On Allegro (BWV 998), the third 16th note of measure 87 should be G sharp and not G Natural (see photo).
This is not difficult to understand if we know how to sing music in all different keys with movable Do. So in measure 87, while the music is in A (D's dominant), if we sing this in Do (with pitch of A) it is Mi Do Ti Do Re Do. Bach really doesn't have to add a sharp sign (on G) in the key signature, because G has to be sharpened, it is the 7th degree of the A major scale - a leading note leads to the key note A, as if we play Ti (B) to Do (C) in a simple C major scale. It has to be semitone; G natural has no relation in the key of A. G# does. This is also evidently true if you play measure 75 of the Fugue, where the Allegro was developed from (see photo), to know the process how Bach moves D major to A major with all G sharpened, and it is not uncommon Bach used one or two measures of the dominant key before returning to the home key - D. (Originally in bE)
In Gigue (BWV 996) - measure 15, the last 16th note of the first 16th notes group should be F sharp and not F natural (see photo).
In the movable Do singing system, it sings Do Mi So Do Do Ti - La as if we play C E G C C B - A in the key of C (relative of A minor) and just like how we play the third measure of Romance de Amore, we sing Do Ti La in the pitch of G, F# and E. Bach use F# as a bridge to run G to the following measure of its subdominant C with that very important connecting note. Once again, the F natural has no relation and is not part of the G major scale. F# is. Look at how many times Bach used this in the Bourree. In the Gigue, the last two notes of lower voice at measure18 also demonstrates his intend of using G - F# (There was no natural sign on F) and landed with E, the first quarter note at ms. 19.
Same goes for measure 11, first quarter note (G) and eighth note (F#) to second quarter
note (E). F is and must be played as F# since there was not natural sign.
We need to watch out for the leading note (7) when it is descending on Bach's music, as well as how Bach sharpened the subdominant note when it is ascending and overlapping that note as a leading note (bridge) to take it to and change to the dominant key, as we found in many sections in Allegro (BWV 998). Measure 18 through 19 of the Allegro is a typical method how Bach (in the lower voice) change to the dominant A by sharpened three bass’s G (D’s subdominant) to used and implied that note as the leading note leads to the new tonic A (dominant of D). And the transition is so smoothly completed with a sharpened subdominant.
I honestly believe that movable Do (C) singing is the best way to identify the notes. When I took the Theory exam at the age of eleven, all the exam of Grade 4 and 5 consist of movable Do singing, the examiner only plays the key chord once (on the piano), and the rest is for us to transpose and sing using the C scale degree with the pitch of the given key. It all makes lots of sense now. I received the Diploma of Theory and Performance at the Royal School of Music at eleven, but the real priceless thing is the learning process that allowed me to understand them in the musical sense inside out. Music is not just a beautiful palace to be seen from the outside, but really takes me inside the palace to enjoy the treasure of our dearest friend, J. S. Bach - a great composer with all notes so precise and perfect. If we don't understand his music, it is not his fault, it is ours.
Measure 87 of the Allegro (BWV 998), please also check measure 75 of Fugue below.
Measure 75 of Fugue (BWV 998) where the Allegro was developed from -- The correct sharpening of G to serve as a leading note leads to a dominant key -- A. (without changing of key signature) All G were sharpened - Bach and Vivaldi's trick to change home key to dominant key (D - E7 - A) through G#, a very important kicker for the magic show. Any G natural in this measure will be a disaster. If we sing the 16th notes group of the inner voice with movable Do (in the pitch of A), it sings MI Do Ti Do, Fa Do Ti Do - There shouldn't be any note in between A and G# as if we sing C (Do) to B (Ti) in C major. So if we play those 8 notes of 16th in C Major, they are: E C B C, F C B C. The measure 87 of the Allegro should be the same accordingly.
Measure 15 - Gigue (BWV 996)
Please also check G - F# - E of the lower voice from the last two notes: G (quarter), F# (8th) of measure 18 to E (first quarter note) of measure 19. No version of any transcription or arrangement indicates the lower F# should be played as F natural (Last 8th note in measure 18).