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For example, if the key signature on the left doesn't have any sharps or flats then it could theoretically be in either the C Major scale or the A Minor scale. How do you determine which one?

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If it starts on C and ends on C, it's probably in C. And vice versa.

If it has lots of A minor and E minor chords, it's probably in Am (E is the dominant). Likewise, lots of Cs and Gs implies C Major.

There are a few other indicators — for example, the harmonic minor scale raises the 7th one semitone (as Jenny mentions) and the melodic minor raises both the 6th and 7th while ascending — but largely speaking it doesn't matter. Frequently, they are two names for the same thing. Often people say that minor keys sound sad and major keys sound happy or triumphant, but this is nothing more that psychological tricks (see Why do minor keys sound “sad”?).

You can use C Major to write a sad song, and you can use A minor to write a happy song. In the end, the only way to know what key is being used is to ask the composer. The intent is the only thing that unequivocally distinguishes a major key from the minor with the same key signature.

  • 1
    Actually most pieces in A minor are more likely to have more chords in E major, while pieces in C major will tend to have E minor chords. Another common sign of the minor key is the raised leading tone. – phoog Mar 15 '16 at 9:16
  • @phoog: To further complicate things, it's not uncommon for songs to use chords that would be common in both relative major and minor. Hotel California, for example, is in two sharps, and features the Bm, Em, and F# chords expected of B minor, but also includes the A, D, G, and E chords expected of D major. – supercat Sep 8 '16 at 16:57
  • @supercat sure, spending time in the relative major or minor is pretty normal. But are there many F# minor chords in the song? Those would be more likely in a piece in D than in one in Bb minor. Of course, a song in a major key can also have a secondary dominant on the relative minor, a minor chord on the third scale degree of a major key is far more common than a minor chord on the fifth scale degree of a minor key. (Also, are those E chords really E major? The normal ii chord in D would be E minor.) – phoog Sep 8 '16 at 17:03
  • @phoog: The verses are Bm F# A E G D Em F#. The first E is major; the second one is minor. – supercat Sep 8 '16 at 17:37
  • @supercat ah, chromaticism. – phoog Sep 8 '16 at 19:04
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Minor keys will usually have the 7th note raised. E.g. in A minor the 7th note is G, so you will see Gsharps throughout the music

2

If i want to determine the key of a song or part of a song. I'm looking for dominant chords and where those resolve to. If i see a G7 (and especially when it resolves to C chord) - then i suspect C major. For a minor often this will be E7 (in place o Em7) resolving to Am chord.

This E7 is dominant chord from harmonic minor scale and is in common use.

This is not a 100% sure method, but i would say 90% :) Whether You need to determine key for jazz changes or for the pop tune.

P.S. Looking at the last note in melody can be another tip

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In C-Major, the "main" chord of a song is usually the C-E-G chord, and that's usually the beginning and ending chord of a song. The other two most common chords in simple tunes are C-F-A and B-D-F-G. If the song uses those chords, then it's probably in C major.

If it uses a minor third instead of a major third in the first two notes of the primary chord (3 half-steps vs. 4 half steps), then it's minor. Play these on the piano and you'll hear the difference: C-Eb-G is minor, and C-E-G is the major chord. You can transpose up to A to make the comparisons there, too.

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You look at the score. Between the Clef and the Time Signature there will usually be some sort of indication of the key. If there is no sharps or flats in between the clef and the time signature then you are either in C Major or a minor. (The Major key with no sharps or flats)

Then you need to look at whether there are any notes raised in the piece. The leading tone of minor keys is always raised by a semitone. So if you see a G# you need to think a minor.

  1. Think about the sharps they give.
  2. Determine the Major key that consist of those sharps.
  3. Determine the relative minor of that Major key.
  4. Look for the leading tone of the minor key.
  5. See if it is raised or not.

And remember the note that could be raised could be a flat that is raised to the natural note. It could be a c that is raised with the use of a sharp or it could also be a note with a sharp raised to a double sharp. So do not just assume if you do not see a sharp that there is no notes raised.

  • Unless I am listening to Bach or a music genre besides classical music, what I usually hear and what I prefer for flat keys is natural minor. A minor and sharp keys are the keys in which I commonly hear harmonic minor. But rarely will I hear a B in a C minor piece unless the piece is modulating to a key with a B instead of a Bb in its scale(so like C minor modulating to C major for example). Similar story for A in C minor – Caters Sep 1 '18 at 17:58
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You're kind of overcomplicating this. The best [and easiest] way to find out whether it is C major or A minor is by looking at which note starts and ends the song. Sometimes, this doesn't work and you might feel a little lost. You need to look at which note is within the song more frequently then the other. That should help. I don't know if this is helpful, but thought I'd put my two cents in. :)

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