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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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Tim H
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miniHessel
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    You might find [this question about naming intervals useful](https://music.stackexchange.com/questions/60771/defining-intervals) –  Mar 07 '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 07 '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 07 '18 at 11:09
  • @Tim -- Oops. Yes, a little scary that my typo got 3 consenters! –  Mar 07 '18 at 11:14
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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. –  Mar 07 '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 07 '18 at 13:18
  • _the key name needs an accidental_ **Note:** When a **#** or **b** is part of the key signature, it's not called an _accidental_ . An _accidental_ is just that: An 'accident' - it really 'shouldn't' be there it that key. – Stinkfoot Mar 08 '18 at 02:02
  • I'm noticing a possible flaw in the counting down. Are you using inclusive counting (including the root as the first number)? – skinny peacock Mar 08 '18 at 15:15

3 Answers3

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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.

Tim H
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  • 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 07 '18 at 09:23
  • 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 07 '18 at 09:31
  • I have another question then, how do you know when to use for example G# instead of Ab? Take for example B major, the relative minor key to B major is G#, but why not Ab? – miniHessel Mar 07 '18 at 09:31
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    Because Ab isn't technically a minor third below B. It's an augmented second. – Tim H Mar 07 '18 at 09:33
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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. –  Mar 07 '18 at 09:58
  • Could one identify which notes are accented in a key, just by looking at the key signature? That's probably not normal, but if I looked at the key signature of B major, I can see that C♯, D♯, E, F♯, G♯, and A♯ are all sharps. That would also mean, that if you know your scales, you could just check which notes are sharps, then you would immediately see that this is a B major key "signature"? – miniHessel Mar 07 '18 at 10:03
  • @miniHessel -- that method is not going to work for, e.g. F Major, which has one flat: B♭. You just need to know the key signatures; the easiest way is to memorize the number of sharps and flats for each key. A useful tool is the [Circle of Fifths](https://en.wikipedia.org/wiki/Circle_of_fifths). Learn this (it has other applications, too) and note that C at the top has no sharps or flats, G to the right has one sharp, next D has two sharps, ..., to the left of C, F has one flat, B♭ has two flats, and so on. –  Mar 07 '18 at 10:19
  • Still, looking at the key signature of F major, would tell you that the only note that is accented, is the Bb. So you can use a key signature to check which notes is accented in a key, I guess. – miniHessel Mar 07 '18 at 10:48
  • Then we agree. I just think it's awesome to see how everything connects. – miniHessel Mar 07 '18 at 10:54
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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 07 '18 at 11:20
  • @miniHessel - as Tim mentioned above : Learn about intervals and learn the circle of 5ths and how key signatures work, (or at least memorize them) . Shortcuts without fundamental understanding end up causing you more confusion. The basics are not hard at all, and once you understand them, you will never be confused - it's worth the effort if you have a sincere interest. – Stinkfoot Mar 08 '18 at 02:05
  • I am reading The Complete Musician, so I have touched upon circle of 5ths and intervals. It's a journey though, so far away from understanding everything. – miniHessel Mar 08 '18 at 07:53
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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

Timinycricket
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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.

MCMastery
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  • 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. –  Mar 07 '18 at 17:35