Many advancing musicians are familiar with the harmonic and melodic minor scales but do not know how they came to be and why. Because music theorists could write volumes on the topic, this article will keep it short and sweet. If you take the A natural minor scale (from A to A, all white keys on a piano) and sharpen the 7th degree (G-G#), you now have the A harmonic minor scale.The sharpened 7th degree (G#) now acts as a leading tone to the tonic A.

You can think of the notes as planets. Certain notes in any scale (particularly the tonic note) have something like a gravitational pull on the notes around them. The shorter the distance (ie 1 semitone instead of 2) between them, the stronger the gravitational pull. In the A natural minor scale, the note G sort of wants to go up to A. In the A harmonic minor, the G# REALLY wants to go up to the A. The sharpening of the 7th degree also gives us the traditional V-i or V7-i cadence (E-Am or E7-Am).

The concepts of voice-leading and counterpoint in polyphonic music can be traced back to early vocal music. The emphasis here was on maintaining independent and interesting melodic lines while creating a strong, smoothly connected harmonic framework.

Up to and during the Renaissance period, harmonies considered dissonant were usually avoided (and if not, the dissonances had to be resolved). However, harmonies were not (and are not) the only things that could be considered dissonant. Certain melodic intervals (any augmented or diminished intervals) were avoided.

With the harmonic scale, we run into the dissonant melodic interval (aug. 2nd) between the 6th and (raised) 7th scale degrees. To ascend to the leading tone more smoothly, often the sixth degree was sharpened as well. This is what we now know as the (ascending) melodic minor scale. Essentially the specific scale was created out of a need to approach the leading tone without leaping over a tone and a half to get there. As anyone who has practiced scales "conservatory style" knows, the melodic minor scale ascends with a raised 6th and 7th degree and descends on the natural minor. There are a couple of reasons for this.

First, if you're descending from the tonic, there is no need to keep the 7th degree raised, as it has no leading tone functionality. Second, if you descend a (fixed/jazz) melodic minor, it sounds a LOT like a descending major scale. This can cause tonal ambiguity. In actual use by composers, rarely is the melodic minor used in its sharpened-ascending, natural-descending form. It is really just a smoother way to get from point A to point B (or point Bb). The use of (and induction) of leading tones is a very interesting area of study for advancing musicians.