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I am a self taught keyboard player, and thus humbly request for guidance.

While trying to learn Preludio in C Major (also, Prelude) by J S Bach, I was trying to note down the chord each bar has been broken down into, for ease of reference and for learning new chords. Since I only know the basic chords, I was using this chord identifier to help me with seeing patterns and identifying the chord in each bar. I found that there's some dispute in the chord of bar 12. There are two chords which are made up using the same set of notes (G A# E C#) as in bar 12: Edim7 and Gdim7.

Upon further research, I discovered that the placement of the "E" note was making all the difference.

  • When the E was placed before all other notes, it was being labelled as Edim7.
  • When the E was being placed at the end of all other notes, it was being labelled as Gdim7.

My question is, what difference does it make? The sound produced by both these variants seems similar to my ear. Why does this difference arise? Are there other cases like this where two different chords constitute the same notes, and how do I identify which one it is?

5 Answers 5

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The chord finder tool is wrong

The chord in the Bach in C#dim7. This is because the chord in m. 13 is D minor, and the primary function of C#dim7 is to lead into a D chord (major or minor). This is the importance of spelling for enharmonically equivalent chords: the spelling helps indicate the chord's musical function.

The problem with the chord finder tool is that it doesn't distinguish between enharmonically equivalent pitches. Although dim7 chords share pitches, they are distinguished by spelling to help clarify their function in music.

  • C#dim7: C#-E-G-Bb
  • Edim7: E-G-Bb-Db
  • Gdim7: G-Bb-Db-Fb
  • Bbdim7: Bb-Db-Fb-Abb

A similar analysis holds for m. 14. The chord there is Bdim7, which is determined by the following chord: Cmaj. Again, this is a typical function for Bdim7 and is reflecting in the way the chord is spelled in the score.

Augmented triads are another example of enharmonically equivalent chords distinguished by function and spelled to reflect that. Caug, Eaug, and G#/Abaug all contain the same absolute pitches, but operate with different expectations. Caug might typically be followed by Fmaj or Amin; Eaug by Amaj or C#min; and G#/Abaug by C#maj or Fmin.

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  • Isn't it a (broken-up) C major chord in Bar 15, not a C minor chord?
    – Dekkadeci
    Jun 10 at 15:57
  • @Dekkadeci Indeed it is. Thanks. I've corrected the post.
    – Aaron
    Jun 10 at 15:58
  • Shouldn't that be Abb rather than Ab in the Bbdim7 spelling? Jun 10 at 16:32
  • @BrianTHOMAS Yes. Thanks. Fixed now.
    – Aaron
    Jun 10 at 16:38
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There are only 3 possible diminished seventh chords in 12-tone naming. The best way to look at things is to note the way the chord progresses. The 4th stacked minor third lands on the octave of the lowest note.

One usually notates that chord by its function (where the chord leads or actually, what's the next chord?). E diminished 7 is 3 stacked minor thirds: E-G-Bb-Db whereas G diminished 7 is G-Bb-Db-Fb.

In many chord progressions, the diminished 7 is used as a minor ninth with the root missing. Sometimes this helps the analysis. The E dim7 could be treated as an C minor ninth with a missing C and the G minor ninth as an Eb minor ninth with the Eb missing. (In a string of sequences, this is not usually the case, but in most other cases, it seems to be useful.)

There are other cases (A minor 7 and C with a 6th added or the German Sixth). In the 19th century (and probably earlier), composers would lead into a chord as if it were one version and leave as if it were another. This is particularly easy with diminished 7ths. One could (and someone probably has) lead into an E dim7 chord and leave as a G dim7.

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  • Anyone assuming that in Bach's time, the 4 diminished chords that used the same set of keys were considered identical, is wrong. When you have a key instrument with separate flat and sharp keys, you hear how much difference there is between the chords.
    – yo'
    Jun 11 at 7:48
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There are four different names for diminished seventh chords - each reflecting a root. Since diminished seventh chords are 'symmetrical', in that each note contained within is three semitones away from the next, etc., it means that your diminished chord will have four notes - E, G, B♭, D♭, or their corresponding enharmonic names.

Generally, the lowest note played will dictate which name will be given, or it's dependant on the prior and next harmony, but in a lot of cases I've seen, any name (from the four notes) is given. Since in any key, there's only one diatonic diminished seventh chord, maybe that's the safe name to give it - rooted on the 7th note from that scale.

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Yes, Gdim7, B♭dim7, C♯dim7 and Edim7 all use the same 4 notes. Sometimes it's pretty clear which name would be best, sometimes not so much!

How would you analyse the chord at bar 12?

enter image description here

It's Bach, so we can pretty well assume it's tonal, functional harmony. It's preceded by a G major chord, followed by a F-rooted chord (in this context I'm tempted to think of it as F6 rather than a first-inversion Dm, but please yourself). Anyway, our chord seems to have a dominant function in relation to the chord that follows. That might suggest Edim7, as VIIdim7 of F. But it's also plainly a modification of the preceding G major chord, so would it be better to label it Gdim7? Functionally, we could even call it a rootless C7♭9.

There's no right answer. You should understand the discussion. Great piece, isn't it, despite its deceptive simplicity.

(Do you see why your question renaming the notated B♭ as A♯ made me flinch?)

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  • 1
    If renaming the Bb as A# made you flinch, the why does it not bother you to rename the C# to the Db that would otherwise be required by your interpretations as Edim, Gdim, or C7b9? (Also, in each of those cases, the Db would be expected to resolve to C.)
    – Aaron
    Jun 10 at 13:52
  • Because in this case the C# resolves up to D, the Bb down to A. And Bach chose to spell them that way. Changing his spelling of the Bb to A# seems merely perverse. Jun 10 at 15:03
  • @LaurencePayne I am sorry if my mislabelling of B♭ as A# seemed 'perverse' to you, but like I said, I am learning to play this instrument unguided. I am not aware of the correct and appropriate lexicality of different concepts, but I am trying to make myself aware as much as possible through the internet. My choice of spelling was based on the comfort of habit. Additionally, thanks for the answer, it was very helpful! Jun 10 at 15:39
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In a full diminished seventh chord whose root and seventh are both present, there will be exactly one pair of notes with consecutive letter names. Regardless of where they appear in the chord voicing, the upper of those two notes will be the root, and the lower of them will be the seventh. Because the chord in bar 12 contains both a Bb and C#, and because B and C are consecutive letters, the only kind of diminished seventh chord it could be is a C# diminished seventh (note that some kinds of chords other than diminished seventh may have a pair of non-root notes with consecutive letter names. A Csus chord, for example, would contain both an F and a G. If something is a full diminished seventh chord, however, notes with consecutive letter names will be the seventh and root.

A similar principle applies with augmented triads when the only notes present are those of the triad, except that here one should look for a pair of notes that are three letter names apart, such as D and G, or G and C. The note which is three letter names above the other is the root, and the other is the raised fifth.

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  • A Dsus contains F and G? No, it doesn't. The whole point of sus is the 3 gets taken out, there replaced by the G.
    – Tim
    Jun 11 at 16:49
  • Sorry--I meant a Csus. Corrected. (thanks)
    – supercat
    Jun 11 at 16:56

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