C major scale has the notes C D E F G A B C
A natural minor scale has the notes A B C D E F G A
They share the same chords too : C Dm Em F G Am Bdim C
Suppose we are playing the notes … G A B C D E F G A B C D E F G A… notes without sharps and flats.
We can think that since there are no sharps and flats, the melody is in the scale of C major, where C major chord will be root chord.
But then, Am will also have the same notes – no sharps and flats.
The song in a minor scale and a song in its relative major will have very different moods, attitudes, altogether.
How do we find, just from theory, if a song is in a major scale or its relative minor?
One should look for a particular note, which will clear the doubts. The minor scale song harmony, most probably will have just one note different from the major scale.
Even when we feel that a song is in minor, how do we make sure? A strong evidence is needed. We may find some to prove the case.
Let us take the song Hotel California.
Bm Em D A G … and a few more chords.
D major scale has the following chords : D Em F#m G A Bm C#dim
All the chords of the D major scale should be valid for B natural minor scale as well. (relative minor of D is Bm. has the same relation C has with Am)
The chords seem to be there in D major scale. How do we know if the song is in D major or Bm?
Let us observe the premises more carefully. Are there any more chords played in the song.
What is the second chord played after we start with Bm? It is an F# major . Major, not a minor. While in the chords for D major scale, we find F# minor.
That single chord evidence turned the story.
F# major has the notes : F# A# C#
F# minor has the notes : F# A C# (all notes in the D major scale or B natural minor scale)
Which note is different? The A.
The 5th note in the D major scale. Or the seventh note in the minor scale.
B minor has the following notes : B C# D E F# G A B
We observed that one of the notes is different in the song. That is the A note.
A is the seventh note in the B minor scale.
Changing A to A# – sharpening the seventh note of the natural minor scale – gives us the Harmonic minor scale
To begin with, musicians were not happy playing the F# minor to B minor movement when the song is in B minor. That is the 'five to one' motion, but the five is a minor.
To have the tension, suspense and then the release (perfect cadence), minor chord of the fifth was not enough, one needed the major chord of the fifth. (the dominant seventh could be even better, but let us stay with triads).
Taking the example of B minor, the fifth chord F#m to F#major, was changed, thus raising the seventh note of the natural minor scale by a sharp.
This new creation, the harmonic minor, promised them enough excitement in the harmony.
If a melody is in the major scale or its relative minor can be seen by noting if in the possible major scale, the 5th note is sharpened or not.
In our example, the A was made A# (F#m to F# major chord). The fifth of D major scale was sharpened. The B harmonic minor scale was used.
When a song is in a minor scale, chances are the fifth of the root chord will be a major or a dominant seventh chord. And to make this change, the seventh note of the natural minor or the fifth note of the relative major scale will be sharpened.
That will give away the secret and let us know that the song is in a minor scale.
When learning western music theory on paper, we follow the same root. Find which notes are used. See which major scale it falls in. Then see if you find that the fifth note is sharpened. If yes, decide that the melody is in the relative harmonic minor.
Dm C A# – these three chords in that sequence, has been used quite often (sultans swing to mind).
Those chords are from the F major scale.
We may feel that the chord is in the Dm. How do we make sure.
Usually after those three chords, comes the A major chord.
The A chord in the F major scale is A minor, not a major.
what is the difference in notes between A major and A minor?
A C# E and A C E
The C note is the fifth note in the F major scale. And we see that it has been sharpened.
C is the seventh note in the D minor scale (relative minor scale of F major). C when sharpened to C# gives us the D harmonic minor scale.
The presence of C# found from the A major chord tells us that the song is in Dm and not F major.
I rest my case.
For a complete easy to grasp resource on understanding and using chords on your own, see The Chord Code