The three minor scales, and the problem each one solves
Natural, harmonic and melodic are not three matters of taste; they are three answers to the same structural defect.
Major has one scale. Minor has three. This is usually presented as an eccentricity to memorise, and it is the opposite: the two extra scales exist to fix a concrete problem with the first one.
The natural minor
The natural minor scale is the major with three degrees lowered: the third, the sixth and the seventh.
A natural minor uses exactly the same notes as C major. It is the relative minor: same key signature, different centre of gravity. What changes is where the music rests.
The defect
Play the seventh degree of the natural minor scale, G, and then the tonic, A.
It does not pull. The distance between them is a whole tone, and the ear feels no obligation to continue. Compare with the major, where leading tone and tonic are a semitone apart:
That semitone is what makes a cadence sound like an ending. Without it, the fifth degree becomes a minor chord, in A minor, and the return to the tonic loses force.
The harmonic minor
The solution is direct: raise the seventh degree a semitone, and that is that.
Now G♯ sits a semitone from A, the fifth degree becomes major, and the cadence works:
instead of . All tonal music in the minor mode, from the Baroque to choro, depends on that alteration.
The price appears between the sixth and seventh degrees: F to G♯ is a tone and a half, an . Sung, that augmented second sounds out of place, and it is the reason the harmonic scale is excellent for harmonising and awkward for melody.
The melodic minor
The third scale removes the awkwardness without bringing the problem back: the sixth degree is raised as well, closing the gap.
Major sixth and seventh, minor third. Ascending, it sounds almost like a major scale with the third lowered, which is exactly what it is.
When to use each
Nobody chooses one minor scale for a whole piece. The three coexist, and the passage decides:
| Situation | Scale | | --- | --- | | Melody descending, no cadence in sight | Natural | | Dominant chord, preparing the tonic | Harmonic | | Melody ascending towards the tonic | Melodic |
A piece in A minor uses G natural, G♯ and F♯ in different bars, without changing key. The signature records the natural scale; the alterations appear as occasional accidentals.
What to take from this
They are not three competing scales. They are one scale and two adjustments: one to give the cadence force, the other to remove the bump the first one created. Understanding the problem each one solves makes memorising the three unnecessary.