Why use natural minor for melodic minor descending?

The natural minor developed for some complicated reasons - to understand why, you have to know the history.

First we had the natural minor scale. That one's obvious enough - it used only natural notes (for A natural minor), so in the early days of Western music, when we only had seven notes to work with, that was all we could use. Although the natural minor wasn't one of the church modes, Heinrich Glarens (the guy who created the concept of modal relationship in 1547) wrote of it as one of the secular scales - it was probably used in folk music throughout the early middle ages, and definitely used before the baroque era started.

In the early middle ages we started getting into harmony, and we also discovered there were notes in between the seven natural notes. Those two things combined to create cadences, like the V7-I authentic cadence. (You can have a V7-I without accidentals as G7-C, but the two developments happened pretty much simultaneously, and both were needed to form the cadence in minor keys).

If you're working in a minor key, the V-I cadence doesn't have the same tension with natural notes - you'd have Em7-Am, and the Em7 doesn't have tension; unlike a G7 chord, there's no tritone. To include the tritone in an Am cadence required using G#, and the harmonic minor was created.

The melodic minor took a couple hundred more years to develop... the reason it was needed is pretty straightforward: the melodic minor is hard to sing. It's easy to sing a half step or a whole step, but an augmented second (F-G#) is a lot tougher. Raising the F a half step made it a lot easier to sing.

But if all we did was raise the F, we've got trouble: if the melody spends much time away from the bottom half of the scale, it stops sounding minor. The ascending melodic minor is the same as a major scale, except for the b3. So unless you're including the b3 pretty often, you get a major sound - not what composers were looking for.

So we needed a scale that included G# (to get the desired harmony), used F# (to make it easy to sing), and sounded minor even if you were in the upper half of the scale. Combining the altered harmonic minor with the natural minor did the trick. And although it's taught as 'raise the 6th and 7th going up, but not going down', that's just a rule of thumb - there are lots of instances throughout music history that include G# or F# as part of a descending sequence in Am, or the natural notes as part of an ascending one. But to keep it sounding minor, you need balance, and the rule of thumb is a very easy way to make sure you get it.

The underlying chords can be much more complicated. Because you now have a choice of what to use for the 6th and 7th scale degrees, you can harmonize to many more chords than you can in a major scale. Chord progressions can include augmented chords, or more diminished chords than you can have in major keys. (The harmonic minor also brought about innovation in harmony: extending the harmony leads us to altered dominant chords, as E-G#-B-D-F = E7b9; later developments, like bebop, actually depended on these developments that happened 400-600 years earlier)

Over a vamp you can do whatever you like. Because vamps eliminate harmonic tension, you can pretty much use any scale at any time - that's why they're so good for practicing various scales. The thing that makes minor keys different is their richness - because we've got choices that lead to greater dissonance than we can get in the major scale, we can get away with a lot more...

1 = always good in any key, because it's the tonic
b2 = in the phrygian scale, one of the minor modes
2 = in most of the non-phrygian minor scales
b3 = in every minor scale (that's what makes it minor)
4 = in most minor scales
#4 = in the gypsy minor scale
5 = in every minor scale except the Locrian. And since it's enharmonic to #4, it still won't bust up your minor sound.
b6 = in the natural minor
6 = in the melodic minor and the dorian
b7 = in the natural minor
7 = in the harmonic and melodic minors, and some others

That's every note except the natural 3rd. You can even use that one (with care) and there will be times you can make it fit -for example, in the 4th measure of a 12 bar blues in a minor key to add a tension to a tonic chord.

Nice post. Worth making sticky or tagging as a FAQ or something.

Oh, yes. I'm printing this out and putting it in my studies folder. Excellent, many thanks Noteboat. 8)

I had to look at the wiki for 'augmented second' - sure looks like a minor 3rd to me! :lol:

Nah, minor 3rd would be F-Ab.

You can't call Fs minor 3rd G# because you have G as the major 2nd in that minor scale, so you call it an augmented 2nd because you raise the major 2nd by a semitone. They sound the same though. ^^