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The top line is soprano and the bottom line is bass. Is there supposed to be some sort of harmonic function used by Bach by the use of C#?

The key signature is Bb Major, though this passage could also either be F Major or G Minor.

  • The C♯ is implying probably A7 or C♯ diminished inbetween the first bar's C major or a minor and the second's predominant function chord. What do you mean "Is there supposed to be some sort of function used by Bach"? Do you mean "what is the function of the chromatic note C♯ here"? – user45266 Sep 26 '18 at 0:16
  • With "used by Bach" do you mean "according to the rules for four-part harmony". Also: it would help if we'd know in what key this fragment is written? – Tim H Sep 26 '18 at 8:57
  • We do not have all the context, but it could be a secondary dominant: A7/C# resolving to D or Dmin. – Karlo Sep 26 '18 at 9:31
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This isn't really a matter of Bach 'harmonising a chromatic'. The melody doubtless came first. He's harmonising it WITH a chromatic.

A sharpened chromatic note will generally have the function of a leading note, the third of a dominant type chord. In this case the C# seems to be the third of A7, the dominant of D minor (assuming no key signature).

Maybe the music continues in D minor for a bit, in which case we could call a modulation. I suspect however that it's a very fleeting tonicisation of D, not worthy of that label. The chromatic note is just being chromatic. Adding colour between a pair of diatonic chords rather than leading us somewhere new.

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In this instance, it being Bach and the C# is in the bass, I would guess that this chord would be a first-inversion A7 leading into a D major or minor chord. I doubt that it is a C# dim because in Bach-style chorale writing that chord would normally be in 1st inversion to avoid undesirable parallel movement. If the chord is a fully-diminished 7th, the inversion doesn't matter but that chord really came into use later than Bach.

In any voicing, the leading tone is normally followed by the tonic by half-step. In a secondary dominance like this one, the C# is acting like a leading tone into the D, so these chromatic movements are usually by half-step. Bach could have chosen a different voicing, but no matter what voice in which the C# was placed, it would be followed by D since it is "tonicizing" that chord. In this instance, the chromatic movement makes for quite a nice bass line.

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