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I'm writing a piece in the key of C major. One of the chords is a C♯dim7. The question is, do I notate that diminished 7th as A♯ or B♭?

The context is a chord sequence Cmay7 - C#dim7 - Dm7 - Dm/B; the C#dim7 is played with the C# in the bass.

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Aaron's answer is correct if we assume that you are right to call the chord C♯º7. Are you? It could just as well be D♭º7. For that matter, it could also be Eº7, Gº7, A♯º7 or B♭º7, though the flat roots are unlikely (there are other even more unlikely possibilities such as F♭º7 or B♯♯º7).

Which of these is the best choice for the chord depends on the harmonic context. For example, if the following chord is B minor or some other B chord, then it's probably A♯º7. If the next chord is B and the bass note is C♯ then it's in first inversion: A♯º7/C♯. If the next chord is some sort of F chord then it's probably Eº7.

Here are the four most likely possibilities (that is, the ones that avoid double flats). The root is the first pitch given, but the bass note can be any of the pitches:

  • A♯-C♯-E-G
  • C♯-E-G-B♭
  • E-G-B♭-D♭
  • G-B♭-D♭-F♭
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    Yes, quite often it does make more functional sense to have Db in C (bII) than C# (#I). But then as dim7 not so much. Rather we’d expect some maj7 or dominant 7 as some sort of neapolitan like chord. But then C# dim7 suddenly makes sense as A dom7 (i.e. the triple dominant chord). E dim7 would make sense as C dom7, so a secondary dominant on the tonic. g dim7 could be a Eb dom7 as altered mediant. Bb dim7 could be in fact F# dom7 as a tritone substitute of tonic dom7.
    – Lazy
    Oct 13, 2022 at 9:35
  • @Lazy yes, D flat is unlikely. E is more likely. My main point, though, is that if someone isn't sure how to spell the seventh, they ooght to consider whether they're spelling the root correctly and, if it's a symmetrical chord (º7 or augmented triad) whether they've identified the root correctly.
    – phoog
    Oct 13, 2022 at 14:44
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B♭

Because it is a "seventh" chord, it must include a note which is the interval of a seventh above the root. Since the root is a "version" of C, the seventh must be a version of B.

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C dim 7 is spelled C, E♭, G♭, B♭♭. (B♭♭,not A, as a lot of us may think). I start with this standpoint, as C is probably easier for most of us to understand, not because the piece is in key C.

Now, since you've said the chord is C♯; diminished, let's sharpen each of those notes: C♯, E, G, B♭. Not an A name in sight!

Diminisheds are peculiar in their naming, as depending on their roots, and/or their status in the section involved, their notes may well have different enharmonic names - as your question points to. I hope this simple explanation makes sense of it.

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  • Yeah, and we never lie on a tax return! Bbb is indubitably correct. But, particularly in 'songs' rather than symphonies, you'll see a lot of C, Eb, F#, A spellings of Cdim7.
    – Laurence
    Oct 13, 2022 at 12:12
  • @Laurence - true, it took me years to realise my A was in fact Bbb.
    – Tim
    Oct 13, 2022 at 13:13
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    @Laurence The spelling you mention is the correct spelling for F#dim7, which is enharmonic to Cdim7. Which spelling is the appropriate one depends on context, of course. Oct 15, 2022 at 9:02
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    Michael Karcher Indeed. My point is that whatever note is designated as the root, in practice we are often flexible about 'correct' spellings.
    – Laurence
    Oct 15, 2022 at 16:59
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It usually depends on how the notes all resolve (move a half (rarely a full) step from dissonance to (usually) consonance. We almost always see, for example, B♭ resolve to A instead of B while we typically see A♯ resolve up to B rather than to A. In fact, it might be more accurate to rename C♯dim7 as D♭dim7 depending on how most of the notes in the chord resolve.

It's possible that how things resolve won't tell you how to write the chord: suppose you finish the song/phrase on the diminished 7 chord or ... new voices take over the song in an entirely unrelated key, or a single melodic line starts after the dim7. Even if there's no explicit resolution, you can still imagine the chord that would bring resolution to the dim7 to guide the spelling.

If you are composing for piano, this is all academic, but singers and instruments with more tonal freedom often play B♭ slightly flatter than A♯.

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  • A minor correction to your assertion that "B♭ (is) slightly flatter than A♯". It depends on the temperament, of course, but I am not aware of any temperament which has B flat lower than A sharp, all that I know have it the other way around. This stems from the fact that, in just intonation, the major third must be lower than in equal temperament and the minor third higher. This results in two different types of semitones: major semitones (e.g. between E and F, or A and B flat) and minor semitones (e.g. between A and A sharp).
    – cdalitz
    Oct 18, 2022 at 10:52
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It doesn't matter what key you are in. C♯dim7 will always be spelled C♯ E G B♭, because chords are spelled in thirds. And of course that means the diminished seventh of the chord is the C♯ B♭ interval.

The only way the key would make a difference isn't in the spelling of the pitches, but whether or not accidentals are needed to notate it. For example, in C major - with key signature of zero sharps/flats - you need to write accidentals for the C♯ and B♭, but if you were is a key like D minor - with key signature of one flat - you would only need an accidental for the C♯. But, in either case the actual pitches are still the same in both keys: C♯ E G B♭.

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C♯dim7 is C♯, E, G, B♭. A pile of minor 3rd intervals, spelt that way.

Cdim7 is more interesting. The 'correct' spelling is certainly C, E♭, G♭, B♭♭. But in non-academic (and possibly non-'classical') contexts this will often be simplified to C, E♭, F♯, A. There are plenty of Cdim7 chords in popular music, but you won't spot too many B♭♭ notes!

But there's no argument over spelling C♯dim7 as C♯, E, G, B♭. The 'correct' spelling coincides with the 'easy' one.

enter image description here

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    Just to comment: Your last example is a nice F#dim :)
    – yo'
    Oct 15, 2022 at 15:06
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to get a 'to the point' answer you need to add some more info.

  1. What chord you put after "C#dim7"?

If for example you wrote a Dm7, then C#dim7 is the right spelling.

If the next chord is Cmaj7, then Dbdim7 is a better option.

(Modulation is excluded here as you said you are writing a piece in C major)

  1. In which style you are composing?

Even if the harmony is the same for classic or jazz, chords are treated differently, depending on the style

Best

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  • This may have been better as a comment, asking OP for more information. There's no mention of the asked A#/Bb.
    – Tim
    Oct 15, 2022 at 10:49
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The chords are everything about intervals (for the most part). In your case, the Diminished 7th Chord is made up of a Diminished Triad which is stacked up with a Diminished 7th Note (which you may already know). For the correct spelling, you have to workout the intervals correctly. The main basis to identify intervals is that the quality of the interval should agree with its quantity. For example the interval between C to C# makes an Augmented Unison and not a Minor 2nd (which is a common misconception). So a Diminished 7th from C# would obviously be Bb since the quality agrees with its quantity. Also this will not affect the context of your chord progression adversely. In the key of C Major, the C#°7 chord is a secondary leading note chord that travels onto the key of D Minor. So I conclude that mastering your intervals will definitely help you to understand harmony in a better way.

Best of Luck with your Music Theory Studies

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