Take the following example from musictheory.net. If we go by the "cheat" rule where you can take the last sharp, then go one half note up, it will tell us that the major key is A.

Next, if we use the "cheat" rule of counting down a minor 3rd to find the relative minor key, we get F. That's okay, but how do I know when the key name needs an accidental? I would have guessed F here, not F#.

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    You might find this question about naming intervals useful – ex nihilo Mar 7 '18 at 10:01
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    This is one of the many times when theory on its own is greatly helped with an instrument. Playing the A chord, followed by FM will probably tell the payer that they aren't relative at all. Playing A then F#m, the similarity should be heard. Not an answer, merely a comment, constructive, I hope. – Tim Mar 7 '18 at 10:26
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    @DavidBowling - relative minor is a m3 below the major root, but that also makes it M6 above, rather than M3. Scary that 3 people agree with what's there! – Tim Mar 7 '18 at 11:09
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    F is not a minor third below A, F♯ is a minor third below A. It looks like you need a better "cheat" rule: what will you do with the flat keys? Just learn the major key signatures first, then the relative minor is a minor third below (or a major sixth above) the major key: 3 sharps --> A Major; down a minor third --> F♯ Minor. – ex nihilo Mar 7 '18 at 11:15
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    It really is worth learning the key sigs - at least to 4 or 5 # and b. – Tim Mar 7 '18 at 13:18

You know because you have to count down a minor third. A minor third interval consists of one whole tone and one half tone. Between A and G there is one whole tone. Between G and F there is another whole tone. To make the interval between G and F a half tone you have to raise the F by a half tone by putting a sharp in front of it.

You also have an important hint by seeing that the F# is already in the key signature. Which probably is a faster way of getting it right.

  • Thank you for explaining. I messed up on intervals, at least I learned something new! So if you can see the note of the signature accented, that's what it is for the relative minor? If the minor was an A, and in the major the A was flat, that would make the relative minor an A flat? – miniHessel Mar 7 '18 at 9:23
  • Yes, it would (except that A flat minor isn't used). The shortcut works because in the major and the natural (!) minor scales all the accidentals are already in the key signature. – Tim H Mar 7 '18 at 9:31
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    Because Ab isn't technically a minor third below B. It's an augmented second. – Tim H Mar 7 '18 at 9:33
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    Note that A♭ Minor is the relative minor to C♭ Major; not a common key, but a real key all the same. – ex nihilo Mar 7 '18 at 9:58
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    @miniHessel - not sure that 'accented' is a good or accurate term to use here. It sounds like 'accented notes' are the main ones in a key, which they're not. If there was a word 'accidentalised', that may do! – Tim Mar 7 '18 at 11:20

Because it tells you right there that the key has 3 sharps and which notes are sharp. F, G, AND C are sharp.

When you do your "cheat" you land on F which you can see needs to be sharp by the key signature. But also if you knew your intervals you'd know a minor third below A is F# and a major third below A is F


I don't think the other answers caught on to your mistake. The minor 3rd is from the Major key name (A), not the last accidental (G#) which is what you did.

  • A minor third below G♯ is E♯, not F. If this is what OP did (and I am not sure that they did), they got it wrong two ways. – ex nihilo Mar 7 '18 at 17:35

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