Why Are There Three Types Of Minor Scales?

[DonRocks]

It's going to take someone with at least a semester of music theory to answer this question, but (in piano, at least), there are three types of minor scales: natural, harmonic, and melodic.

The "simple" definitions are these:

Start with the major scale, and go down three half-steps to reach the relative minor scale (e.g., C major's relative minor scale is A minor, going down three half-steps: B, B-flat, and then A).

The natural minor will have no accidental notes - an "A natural minor scale" is the exact same thing as a C major scale (with different fingerings, since you have to start on the correct note).

The harmonic minor will raise the 7th note by a half-step - an "A harmonic minor scale" raises the G to an A-flat, producing a haunting, "Arabian" sound.

The melodic minor will raise the 6th and 7tth notes by a half-step - an "A melodic minor scale" raises the G to an A-flat, but also the F to a G-flat.

That's all fine, and everything up until now has been simple definitions; my question is this:

In practice (double entendre on practice), pianists often, perhaps generally, practice two types of minor scales:

The harmonic minor, both going up, and coming down. This makes perfect sense, and it seems like there should be two others practiced: the natural minor, and the melodic minor, but there aren't, and that's what doesn't make sense to me.

The second type of scale often practiced is a melodic minor going up, but a natural minor coming back down. Why is this? Why isn't it the other way around, for example, i.e., why isn't it a natural minor going up, and a melodic minor coming back down? Or, for that matter, why aren't the two practiced separately: a natural minor going both up and down, and a melodic minor going both up and down? Too much weight on the scales, perhaps? (That was funny; you just don't realize that it was funny.)

At various times in my life, I've wondered this, and have never found an answer. Instead of scouring Google, I'm going to put this out there, in hopes that a music theorist might see this one day, sign up for the website, and answer the question. There are at least five members who might know the answer, but like me, they've probably just accepted it as "the way things are," and haven't spent much time questioning it.

Thanks if anyone can help.

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Leonard Bernstein (The Hersch)