Types of minor scales
A natural minor scale has the following interval pattern:
tone semitone
tone tone semitone tone tone
T S
T T
S T
T
If the scale is used with the corresponding key signature, the natural minor scale is written with no accidentals.
For example, in the key of A minor, the natural minor scale is:
A B C
D E F G A'
Sometimes the natural minor scale is equated with the Aeolian mode. This is commonly used in Peruvian and other ethnic music, and by modern Western composers such as Vaughan Williams who have a liking for this sound. But in music written from the 16th to 19th centuries, the chord built on the dominant (fifth scale degree) is almost always a major triad, at least at cadence points; consequently, the seventh degree of the scale must be raised with an accidental to make this possible. The next most important chord, the subdominant, is typically a minor triad.
These considerations of harmony lead to the harmonic minor scale, the same as the natural minor but with a chromatically raised seventh degree.
tone semitone
tone tone semitone tone and a half semitone
T S
T T
S
T&1/2
S
For example, in the key of A minor, the harmonic minor scale is:
A B C
D E F G♯ A'
The interval between the sixth and seventh degrees of this scale (in this case F and G sharp) is an augmented second. While some composers, notably Mozart, have used this interval to advantage in melodic composition, other composers have felt it to be an awkward leap, particularly in vocal music. So, for purposes of melody, either the subtonic is used, or the sixth degree of the scale is raised; either way, there is a tone between these two degrees of the scale, considered more conducive to smooth melody writing.
Traditionally, music theorists have called these two options the ascending melodic (also known as heptatonia seconda, set form 7-34) and descending melodic minor scales:
MINOR MELODIC ASCENDING
tone semitone
tone tone tone tone semitone
T S
T T
T T
S
MINOR MELODIC DESCENDING
tone tone
semitone tone tone semitone tone
T T
S T
T S
T
...but historically, composers have not been consistent about using them in ascending and descending melodies. Just as often, composers choose one form or the other based on whether one of the two notes is part of the most recent chord (the prevailing harmony). Another reason might be the use of the mediant chord, based on the third degree of the scale, which is an augmented triad if the raised seventh degree is used; some composers prefer the use of the major triad and therefore use the lowered seventh degree.
