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I'm trying to list the closely related keys to C♯ major, and here is what I get:

  • C♯ major, the original key
  • D♯ minor
  • (???)
  • F♯ major
  • (???)
  • A♯ minor
  • x (B♯ diminished is not a key!)

For the points that have (???) next to it, I get some keys that are not part of standard usage: E♯ minor and G♯ major. Sometimes I think about turning them into enharmonic equivalents which makes F minor and A♭ major, but they do not seem to fit the definition. Are they close to C♯ major?

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No, they're nowhere near! For a start, A♭ has 4 flats, and C♯ has 7 sharps. How could they be related?

However, if we instead change the notes that sound the same into D♭, then we're talking. D♭ has one extra flat to A♭, so it's quite close.

You mention the diatonic chords that are usually associated with a key - I, ii, iii, IV, V, vi, viio. Those are not really keys but chords that are made with the notes from a particular key. Then you say they're not usual. C♯ isn't necessarily a usual key! Some composers loved it, others never ever used it - except perhaps to write for transposing instruments, but that would be rare to use C♯.

So, basic answer to the question as it stands - a big fat NO.

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Not if you spell it that way, no! But Ab major has four flats, Db major has five. So yes, pretty close!

But it sounds as if you're listing the triads that can be made from the notes of C# major rather than listing related keys.

Accepting that, yes E# minor and G# major are theoretically correct. And you've discovered what happens if you choose a key name with lots of sharps (C# major has 7 sharps) rather than one with rather fewer flats (Db major has 4 flats). You get some correct but somewhat unwieldy spellings!

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It depends what you mean by 'closely related'.

Personally, I would say that one perfectly reasonable definition of a 'closely related' key is one that has many notes in common, regardless of spelling (i.e. treating enharmonic equivalents as equivalent). You can't (usually?) hear the spellings, after all. And C# major actually only has one note different to A♭ Major.

This can be seen easily if we 'spell' the notes in C# major according to their enharmonic equivalents in D♭ major: D♭, E♭, F, G♭, A♭, B♭, C.

This collection of notes only has one note different - a flattened G - from the notes in A♭ Major: A♭, B♭, C, D♭, E♭, F, G.

So by that definition, the two keys are very closely-related.

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    It's down to semantics, somewhat. The question doesn't ask about Ab and Db. It's about Ab and C#. both of which are separate keys with actually no common notes per se. Probably why it was dv. – Tim Mar 20 '19 at 16:27
  • @Tim "notes" can mean different things - I think i've been clear here what i mean. Downvotes are always welcome! – topo Reinstate Monica Mar 20 '19 at 20:20
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    Downvotes with good reasons attatched are welcome. Others just go to prove that there are those with no backbone! Understand what you mean by 'notes'. Hope others do to... – Tim Mar 20 '19 at 20:47
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    @MaikaSakuranomiya - am I the only one on this site who believes that dvs are far more use when accompanied by a reason? Marking something wrong doesn't give much (if any) of a clue to the answerer as to how to improve the answer - but then only in the eyes of the dver. Surely a comment is more intelligent? – Tim Jun 16 '19 at 13:36
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    @MaikaSakuranomiya regardless of whether downvotes come into play, disagreements are often an excellent opportunity for learning. So yes, if you're able to specify what you disagree with here, it might be useful. – topo Reinstate Monica Jun 16 '19 at 13:44
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If you try to identify the keys related to C# ... all you have to do is to find the related keys of C and then ad a sharp to the related keys of C and you've got the related keys to C#.

As the dominant key of C is G -> the dominant key of C# is G#

So you'll find that Ab is no related key of C#.

The key immediately clockwise is the dominant key of the key immediately counterclockwise, and features either one more sharp or one less flat.

https://en.wikipedia.org/wiki/Dominant_(music)#Dominant_key

III of C-major is Em -> III of C#-major is E#-minor.

same game with flats:

A is related to D -> Ab is related to Db

etc.

I think about turning them into enharmonic equivalents which makes F minor and A♭ major, but they do not seem to fit the definition.

In modern notation (pop and jazz and in arrangements for bands or even orchestra parts the enharmonic exchange (equivalents) is quite usual and then the keys are violated as flat keys to be related to sharp keys. Since the well-tempered tuning this points are pure theoretical but it makes sense to respect them and to keep them in mind.

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Yes, Ab major is as closely related to C# major as two major keys can be: it's just one fifth away. Ab major is (in equal temperament) the same as G# major, one fifth higher than C# major.

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  • Hi Scott Wallace. According to the accepted answer (written by Tim), A♭ major (and F minor) are not closely related keys to C♯ major. The standard definition of closely related keys is any other key whose tonic triad is diatonic to the original key, and its key signature does not differ by more than one sharp or flat. C♯ major has seven sharps. A♭ major (and F minor) have four flats, and the A♭ and Fm chords are not diatonic chords to C♯ major. Thus, they do not meet the definition. – Maika Sakuranomiya Apr 15 '19 at 4:46
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    Hi Maika- yes, I get it why my answer was not accepted. I'll just have to agree to disagree with you here. The accepted definition makes sense in terms of notation; that is, how the music is written down. My answer makes sense in terms of how we hear music, which, imho, is more important than notational expediencies, when talking about how closely related keys are. But I guess maybe that comes from not reading music much any more, but playing it. – Scott Wallace Apr 15 '19 at 13:08
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    Yes, but this rule (if it is one) has nothing to do with music as something we hear. It is only a notational convention. The circle of fifths in equal temperament makes C# major and Db major identical. To say that Ab major is "closely related" to Db major but "distantly related" to C# major is using the word "related" in an entirely superficial way. – Scott Wallace Apr 17 '19 at 14:31
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    @MaikaSakuranomiya - 'music has rules' is in itself somewhat nebulous. You probably can see Scott's point of view - where if you were asked to write down a note played, and it could be either G# or Ab, which do you write? Semantics! But from the 'closely related' point of view, given their key sigs, they're quite distant. – Tim Apr 19 '19 at 15:50
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    @MaikaSakuranomiya another rule of music is the rule of enharmonic equivalence. On any 12-tone keyboard, for example, A flat and G sharp are not distinguishable from one another. – phoog Apr 22 '19 at 1:58

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