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when we have the key, for example, A (so with 3 sharps), I think that we mark the notes in this key as sharp (right side of the circle od fifths), except for alterations. And what if we have a chord in this key, but it is a diminished chord Abo = AbCb(B)Eb(D), which is already on the left side of the fifth circle, so theoretically its notes we write (I think so) with flats. But since we write the piece in the key of A, we will write Abo also with the sharps, Abo=G#BD?

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  • Eb is not D, as you call it. Maybe you meant Ebb.
    – Tim
    Commented Aug 9, 2020 at 6:31

5 Answers 5

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Wouldn't that diminished chord be G♯ diminished, in key A?

In which case, its notes will be G♯, B and D. So there's no problem - all those notes are diatonic anyway.

The times notes may be written differently would be when there is a modulation, but generally, in sharp keys it's traditional (and clearer) to stick with sharps whenever possible.

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But since we write the piece in the key of A, we will write Abo also with the sharps, Abo=G#BD?

G#BD is correct. Abo7 has nothing to do in A major while G#o is the seventh degree of A.

(Btw. Ab dim would be AbCbEbb)

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Why you have G# and not Ab in the key of A major

If you're in a key such as A major, you assume a scale with seven different letter-named notes. Each of the seven notes can be either natural, sharp or flat, but not e.g. flat and natural at the same time.

Seven notes in the scale:

  • C : natural, sharp or flat?
  • D : natural, sharp or flat?
  • E : natural, sharp or flat?
  • F : natural, sharp or flat?
  • G : natural, sharp or flat?
  • A : natural, sharp or flat?
  • B : natural, sharp or flat?

If you write a chord called "Ab" something, such as Ab dim, then it implies that the A slot in your scale has been set to flat mode. And that would be confusing from the "we're in the key of A-natural major" point of view.

But you can make your G sharp and still have your A natural.

Let's set the seven switches of this aircraft properly for an "A major" flight mode:

  • C : sharp
  • D : natural
  • E : natural
  • F : sharp
  • G : sharp
  • A : natural
  • B : natural
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The answer to every interesting question is “it depends.”

(First, remember that the circle of fifths is an academic exercise, outside the context of a musical composition.)

In some popular genres, particularly jazz, notators often deliberately and correctly choose notes and chord naming that don't really make sense in classical theory. For example, you almost never see a Cb or Ebb in jazz; you would just see a B or D.

Even in classical music, naming depends on the underlying harmonic progression. You may have a key signature, for example, in A major, but have modulated briefly into another key. In this case, you would generally name chords and notes based on the current key, without regard to the key signature. But even then, the best naming should also consider the “function” of a chord or note. In A major, an A# may be better written as Bb if it is acting as a flat-9th in an extended A dominant 7 (VI7/IV). Or it should be an A# if it is a leading tone to a B minor (II) cord.

In your example, the G#-B-D spelling is what you'd see virtually all the time. It's kind of a stretch but I suppose you also might use Ab-Cb-Ebb if it is acting as a VIIdim functioning to lead into Bbb.

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There are two issues here.

One, as others have said, is that in A major it's almost certainly going to be G♯dim rather than A♭dim. The A♭ is the leading note of A, so it's logically as well as practically G♯. (But also remember, harmonic ambiguity is a feature of dim and dim7 chords. Depending where it leads us, A♭dim just might be a logical naming.)

But your point about spelling for convenience rather than harmonic correctness still stands. Let's take a simple example. Key of C, a Cdim7 chord.We know the correct spelling is C, E♭, G♭, B♭♭. But you'll see this often enough in published music.

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Here's my rule for diminished chords. Spell the root of the chord correctly for its harmonic function. Allow yourself some slack when spelling the other notes correctly would result in something ugly. And some musicians would disagree with even that.

(But this is ONLY for diminished chords, and mostly for diminished 7ths. C♯7 is C♯, E♯, G♯ B, NOT C♯, F, G♯ B. Even D♯7 (it pops up occasionally) gets an F double-sharp not a G♮. And the seventh note of F♯ major scale is NOT F!)

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