There are many kinds of scales used in the music of the world: the chromatic, major, and minor scales common to Western classical and popular music; the pentatonic scale popular in both folk and dance music; the microtonal scales found in the music of the Near and Far East; the modal scales also used in folk music and popular, experimental rock, and dance music; the octatonic and hexatonic scales often used as variants of the major and minor scales by modern classical and film composers; and the exotic scales commonly used by composers and music producers to create unusual atmospheres. Knowing these scales puts the music writer in a very good position because there is a scale for every musical purpose and situation. That knowledge also enables the writer to create music that has a clear sense of harmony and coherence.
Natural Minor Scale
The Harmonic Minor Scale
It differs slightly from the natural minor scale because the seventh note of the scale —G— is sharpened to give note G#. The main reason for the use of the note G# is that it gives a better harmony. Chord progressions tend to be guided between two basic poles—a passive pole represented by the tonic chord and an active pole represented by the dominant chord. In the natural minor mode of A, the tonic chord is Am and the dominant chord is Em. A long time ago, musicians discovered that when the chord of Em is changed to E —that is an E major chord— the cadential progression from dominant to tonic was that much more convincing. This was primarily because the G#—only a semitone below the tonic—rose nice and smoothly up to the tonic.
The Melodic Minor Scale
Ancient Greek musical scales developed from the tetrachord a series of four notes spanning a fourth (for example, C D E F). The scales used today use two such tetrachords—a lower tetrachord and an upper tetrachord. The tetrachord system is very good for understanding and portraying differences of mode, as found, for example, in the difference between the natural minor and melodic minor modes.
The melodic minor mode is different in its ascending and descending forms. Classical composers were dissatisfied with the way in which the natural minor mode lent itself to their purposes. Although beautiful in themselves, the harmonies of the natural minor scale were not incisive enough for the dramatic kind of narrative favored by classical composers. To remedy this problem, the first step was sharpening the seventh, giving rise to the harmonic minor scale. This solved a lot of the problem. Sharpening the seventh yielded a powerful major dominant chord that suited their purposes.
However, in some melodic contexts that exotic-sounding upper tetrachord did not sound right. Composers arrived at a solution by sharpening the sixth as well. The result of this was a smooth stepwise motion up to the tonic—exactly like the upper tetrachord of the major mode. To counterbalance that major feeling, on descent the minor mode was re-established. This means that the melodic minor mode is different in its ascending and descending forms.