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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 6 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
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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

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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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