There are several different kinds of minor scale. I am going to look at three of them, the natural minor, the harmonic minor, and the melodic minor.

Remember in part three, we learnt that the chord based on the sixth note of a major scale is called the relative minor chord, so A minor is the relative minor of C major. We also looked at the A natural minor scale, otherwise known as the A aeolian scale. This scale contains the notes A, B, C, D, E, F, G, A; the same notes as the C major scale, but starting from a different point.

It follows that we can build the same chords from A aeolian as we can from C major: Am7, Bm7b5, Cmaj7, Dm7, Em7, Fmaj7 and G7. Unlike C major, our I, IV and V chords are now minor chords: Am7, Dm7, and Em7, or Am, Dm, and Em if we restrict ourselves to triads.

If you listen to the chords Am, Dm, Em, Am, you may hear a characteristic sound which is reminiscent of Eastern European folk music, or possibly medieval music, particularly from the V chord, Em. That minor V chord presents a problem to composers because it doesn't give the tension of a major V chord. Let's look at why this is.

Looking again at C major, the V-I sequence, sometimes called a perfect cadence, goes like this:

G major (GBD) - C major (CEG)

If we add the seventh note to the V chord and rearrange the order of the notes slightly, we get:

G7(DFGB) - C major(CEG)

If you analyse this sequence, you should notice that the B of the G7 chord leads very satisfyingly up one semitone to the C of the C chord, and the F of the G7 chord leads down one semitone to the E of the C chord. You can hear it even more clearly if you omit the notes G and D: F and B move to E and C. This is part of the reason why the V7-I sequence produces that feeling of tension and resolution.

Now if we go back to the key of A minor, we notice that the third note of the V chord, G (third note in E minor) is a whole tone below the root of the I chord, A. This step of a whole tone does not provide such a pleasant feeling of resolution as the semitone step did in the key of C major.

To solve this problem, musicians somewhere back in the mists of time invented the harmonic minor scale, by raising the seventh note of the natural minor scale by a semitone. In the case of A minor, the natural minor scale becomes the harmonic minor scale when the G natural is raised to G#.

This harmonic minor scale now has I, IV, and V7 chords as follows:

I - ACE (A minor)
IV - DFA (D minor)
V - EG#BD (E7)

Listen to these three chords, in the sequence Am, Dm, E7, Am and you should notice a much stronger feeling of resolution from V-I.

Some songs using harmonic minor I-IV-V sequences include Mary Hopkin's 'Those Were the Days', 'Chim, Chiminee' from Mary Poppins, and the opening track from the Buena Vista Social Club album (or the pastiche version in the recent Ford Focus advert on British television).

If we extend our minor I-IV-V triads to four note chords, we will notice that our I chord becomes ACEG#. This chord has the curious name of A minor major seventh. The 'minor' comes from the minor third note, C, and the 'major' comes from the G# which is eleven semitones above the root, as is the case in other major seventh chords. This chord is not used very often - composers tend to use a straight m7 for the I chord when writing in a minor key, even though that leads to a slight anomaly in that, in the key of Am for example, we would have both a G natural (in the I7 chord, Am7) and a G# (in the V7 chord, E7). A possible reason for this is that the sharpened seventh note is only there for the sake of 'majorising' the dominant chord, so it does not need to be included in the tonic chord.

In part two, we mentioned the vii chord of the harmonised major scale, known as the m7b5 or half diminished chord. This chord is not frequently used in major key songs, but it does have a rather lovely function in a minor key. In C major, the vii chord is Bm7b5, but in A minor, this is the ii chord. We can use this in a ii-V-i sequence as follows: Bm7b5-E7-Am7 Listen to this sequence. We will learn more about ii-V-I sequences - note the capital I indicating major I tonic chord - later.

Turning now to the melodic minor scale: this is unusual in that it uses different notes in its ascending and descending forms. In the ascending form, it has a sharpened seventh note, as with the harmonic minor, and a sharpened sixth note. In its descending form, it is the same as the natural minor. When we work out chords based on the melodic minor, we use the ascending form.

A melodic minor (ascending) contains the notes A, B, C, D, E, F#, G#, A.

So our I-IV-V sequence becomes

ACE - A minor
DF#A - D major
EG#B - E major

I can't think of any examples of songs with a sequence like this. If you can, please mail me!

In part seven, we will look more closely at the V-I change, and how it has been extended into more complex harmonic structures.