The reason that modes come into any discussion of the differences between the various minor scales is that the two most commonly used scales, major and minor, are modes—Ionian and Aeolian respectively. The reason that they are the most common is that composers within recent centuries have found them to be the most versatile.

The problem that composers found with the Aeolian mode (natural minor) that led to invent the melodic and harmonic minor scales centers around the fact that the Aeolian mode lacks a leading tone.

A leading tone is a note a half-step below the key note. After playing around in a particular scale for a moment or two, the tonality of that scale gets established in your ear. Then, if you stop playing on the leading tone, it creates a "drop the other shoe" effect. Your ear wants to hear that key note. That's called resolution. This happens automatically in the Ionian mode (major scale) because the seventh degree of the scale is a half-step below the key note (for example, B in the key of C). But not in the Aeolian or most of the other modes.

Where composers especially wanted a strong resolution was at the very end of a piece of music (or at the end of a verse). You will note that almost all songs end with a dominant chord (often a dominant seventh chord) moving to the tonic chord (e.g., G7 to C). The B in the G7 chord moves to the C in the C chord (on guitar, 2nd string open to 2nd string 1st fret), thus producing the desire resolution. The ear is satisfied. That's called an "authentic cadence."

But if you are playing a song that's in Am (natural), the chords available to you are Am, Dm, and Em, with C, F, and G usable. The dominant chord is Em (alternately, G). The note below the key note (A) is G, a whole step down. There is some pull for resolution, but it's nowhere near as strong as it would be if the note were G#. So to create that pull, composers went out of the scale so they could use an E or E7, both of which contain a G#. In most instances it worked very nicely. Then, to sort of make it official, they invented the "harmonic minor scale" (a chord is a function of harmony rather than melody), which, based on A as the key note, is A B C D E F G# A.

But that put an interval of a step and a half between F and G# — an augmented second, the same sound as a minor third — which gives the scale a characteristic Middle Eastern sound. To alleviate that, they raised the F to an F# to smooth out the gap. But as it happens when you start tinkering with something that ain't really busted, they had to keep tinkering. When they descended the scale, the first five notes you hear are the same as the A major scale, and it establishes a major tonality in the ear. So to alleviate that, they decided that the "melodic minor" scale would go up with the raised notes (A B C D E F# G# A) and come down with the same notes as the natural minor (A G F E D C B A). Got it?

Contrasted to the natural minor which has been hanging around for many centuries, perhaps millenia, the harmonic and melodic minor scales are sort of ad hoc inventions, and relatively recent at that.

So what's the practical application of all this? If you are singing a song in Am, check the song to see if it has any G#s or F#s in it. If it doesn't, you're in natural minor. You would probably use Em as your dominant, but substituting G often produces more desirable results. Sounds modal, which is appropriate for many folk songs and ballads. Is modal, as a matter of fact. Don't worry about the lack of a leading tone. People managed to live without it until a few centuries ago. Try an E or E7 just for kicks. It may clank really badly, but if it sounds good, you might want to use it. I sing a couple of songs where I use Em at one point in the verse, G at another, and E7 at the end. No rule other than "which sounds best?"

Incidentally, if your song in Am has an F# in it, but the G is natural, you're in Dorian mode. The available chords there are Am, D (not Dm), and E, with C, G, and Bm usable. Example:— Joan Baez's version of John Riley. If it looks like a major scale except the seventh note (what would be the leading tone) is flatted (e.g., it looks like the key of C, but it has a Bb instead of B), then you're in Mixolydian mode. Use C, F, and G, with Am, Dm, and Em available, but start it and end it on a G chord. Example:— Joan Baez's version of The Great Selchie.

Music theory gives you a good idea of what's possible. The ultimate test is how does it sound?

Now that I've totally bewildered the hell out of everybody, including myself, I'm gonna go fix myself some lunch.

Don Firth