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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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up vote 13 down vote accepted

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, but largely speaking it doesn't matter. They're 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 are minor keys universally accepted to 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 truly distinguishes a major key from the minor with the same key signature.

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You play it and you "just know" :) – Jeffrey Kemp Sep 10 '11 at 1:23
This is not completely accurate it could start on the Upbeat which usually starts on the dominant. Pieces could also end on a modulated key. – Neil Meyer Oct 29 '14 at 8:44
@NeilMeyer Hence "probably" and not "always". – Matthew Read Oct 29 '14 at 23:57
I severely disagree that "they're two names for the same thing" and "the only way to know what to ask the composer". There's an incredibly clear difference between keys in the vast majority of tonal music, and it's not at all hard to tell the difference. – MattPutnam Jun 18 '15 at 14:56
@MattPutnam Please provide an alternative answer. – Matthew Read Jun 18 '15 at 15:26

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

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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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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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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.

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The kind of chords used in the song tells you which key the song is written. For example, a song in the key of f major will have a lot of f major chords and it usually ends on the f major chord.

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There is nothing here that wasn't already in the accepted answer. – dennisdeems Jun 18 '15 at 15:29

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