The differences between natural, harmonic and melodic minors

Question

I'm studying scales and came across the natural, harmonic, and melodic minors. I understand the patterns for each scale but what other important differences exist between the three?

Answer 1

There are at least some differences in how they're used.

• Natural minor is a kind of default.
• In the common practice era a dominant chord usually contains the leading note, which is a semitone below the tonic, because that note has a strong tendency to resolve to the tonic. So, from the harmony point of view, the 7th degree of the natural minor scale should be raised in these cases, and you get the harmonic minor scale. Using a "flat" leading note makes the music sound modal, and was used both before and after the common practice era (probably also during it in folk music and such).
• Raising only the 7th degree causes the scale to have an augmented second between the 6th and 7th degrees. This is a difficult interval to sing and also sounds like a jump unlike the other intervals in the scale. So, from the melodic point of view, if you need the raised 7th and want to go up melodically, you also raise the 6th degree to make things more fluid.
• Since the raised 7th degree is a leding note you usually go melodically upwards when using it. If you're going downwards you're probably not in a dominant harmony and thus you use the natural minor. Another reason for using the natural minor when going downwards is that if you take the upwards melodic minor and play that downwards it'll only start sounding like a minor after the fifth note or so.

In short, you start with the natural minor scale. You use the harmonic minor scale when you need a dominant chord, and modify it to be the melodic minor if you need the leading note in the melody. This all is of course simplified but should give the basic idea.

Answer 2

I'm going to expand upon the answer already provided just to give some further clarification on the history and usage of each scale.

• The Natural Minor scale is also known as the Aeolian Mode, one of the church modes from the plainchant period before the Renaissance. This particular scale can be created by starting on the 6th degree of any major scale, and playing an octave away from that, keeping the key signature the same. Thus:

C Major = C D E F G A B C
A Minor = A B C D E F G A

This is true for all natural minor scales.

• The Harmonic Minor scale developed out of the practice of Musica Ficta, where performers and composers of chants were beginning to experiment and understand cadences. By raising the 7th degree, performers and composers were able to create additional tension / resolution for a tonal center, which led to a more satisfying result.

• This created a problem however as rules of counterpoint were being developed during the early Renaissance. Moving an augmented interval in any one particular voice was considered incorrect voice-leading as it did not facilitate the smoothest possible voice-leading and was therefore jarring to the ear. The Melodic Minor scale was the solution to this problem.

As the scale ascends, the 6th and 7th degrees are raised in order to dissolve the augmented interval between the normally b6 and #7 scale degrees and create a leading tone that resolves strongly to a tonic. Thus:

A Melodic Minor = A B C D E F# G# A (ascending)

As the scale descends, it becomes a natural minor scale, with the 6th and 7th degrees now moving back to their original formation.

A Melodic Minor = A B C D E F G A (descending)

Hope that helps.