Is there a rational method to find out which minor scale sounds best in a given situation?

Question

By rational I mean a method that doesn't entirely depend on one's subjective aesthetics, but rather one that has some kind of system behind it.

The obvious rule that you should use the ascending melodic scale when ascending and the natural melodic scale when descending I have found far too simplistic for actual use, especially in a more polyphonic setting. For one thing, Bach often uses the ascending scale in descending as well, which gives his harmony a distinctly Dorian feel (which I happen to like).

Sometimes the augmented 2nd in the harmonic scale sounds pretty good; other times it sounds absolutely terrible. Sometimes the scale to use changes by the beat; other times the resulting false relations sound nauseatingly ugly.

I would like to have some tools to reason why one or the other is the case.

Answer 1

There are tendencies in many classical pieces on use of the mutable scale steps (6 and 7). These ideas seem to apply to bass more than to melodic voices.

When the harmony is predominately dominant, the raised version of 6 and 7 are preferred. When the harmony is predominately subdominant, the lowered versions of 6 and 7 are preferred. When the harmony is predominately tonic, the raised versions are used in ascending passages and the lowered version in descending passages.

The only other tendency seems to be that the raised 6 is never used as the upper neighbor to scale step 5. The raised 6 and lower 7 are not used together as this may seem to be a modulation to the major key based on the lower 7 or 3 st. This is the V of III which is a common pattern in minor.

Classical theory generally considers the minor mode as a single entity rather than having 3 (or more) scales. Scale passages are common but scales are not fundamental; patterns of chord and scale usage are the primary stuff of classical theory. The harmonic minor is (classically) an artificial structure as the minor iv and major V chords are common. Melodies using a lower 6th and raised 7th (the harmonic minor notes in order) are common in instrumental music, not so much in vocal melodies as it's supposedly to hard to sing the augmented second. On the other hand, arpeggiated dominant minor ninth chords are rather common.

The classical composers seemed to like the sound of the above techniques. They are only tendencies. The ascending and descending versions of all three minor scale patterns are common even in a single piece. Sometimes these are used simultaneously ignoring any fasle relations (or even emphasizing such relations.)

Personally, I'd suggest just using one's ear. I do tend to be a bit strict with scale patterns in the bass while playing whatever seems good at the moment in the melody.

Answer 2

Pat Martino's "Minor Concept" is an improvisation/composition approach that is essentially based on finding the right minor scale or mode for different harmonic situations -- which is quite similar to what your asking.

I think you'll find this subject explained in a video called "Pat Martino Creative Force 1" which should not be hard to find online.

Answer 3

When lines are moving at a harmonic level direction does seem to be the key factor. Lowered ^6 descending to ^5 and raised ^7 ascending to ^1. Those two lines can be extended back to the longer lines ^1 ♭^7 ♭^6 ^5 descending and ^5 ♮^6 ♮^7 ^1 ascending.

Movement using rhythmic subdivision of the harmony (diminution) will use the form of ^6 or ^7 that match the harmony regardless of direction.

...from an aria in the Bach, Anna Magdalena notebook, in the final three bars, two scalar lines descend in the treble, the first time the harmony is dominant and so the descending line uses raised ^6 & ^7, the line is sequenced, but the second iteration is subdominant harmony so the lowered ^6 & ^7 are used. Notice the cross relationship of D, it's natural in the descent of the scale to the dominant, but the harmony is not yet dominant, then it's raised when the harmony has become tonic/dominant and D# ascends to the tonic. The bass when moving to the final cadence uses the lowered ^6 as an auxilliary to the dominant degree, but the chord is a subdominant ii.

When viewing the harmonic level movement in that piece it's important to note the difference in treatment when the dominant is part of a half cadence versus the dominant moving to the tonic. Notice at the fermata the descending line in the treble, the chord is dominant, but the leading tone (D#) does not ascend to the tonic, because it's a half cadence, it's ending on the domimant. In harmonic terms there isn't a leading tone ascent and so the line is "free" to move in any direction. In the next bar the D# is part of a dominant chord moving in a deceptive progression and it ascends to the tonic. It ascends here, not because ascending lines in minor use raised tones, but because the essential harmonic movement of the progression is the leading tone to tonic.

...from the Bach's Two-Part Inventions, in the bass there is a descending line which is sequenced, on the first descent the harmony is not dominant and the lowered ^6 & ^7 are used, in the second iteration the harmony is dominant and the raised ^6 & ^7 are used - despite the line descending. The essential harmonic tones of the dominant chord are pointed out with blue arrows, the descending line uses the raised ^7 to match the dominant harmony, but it moves freely downward, because it is just decorative motion, diminution of the chord.

The is also an interesting occurrence of the augmented second in the third bar. That interval is supposedly avoided or filled in, but it seems to me there isn't a problem using it when the line is a decoration of a dominant chord, especially descending.

The end of bar 2 and all of bar 3 are an elaboration of the dominant chord. Within that dominant harmony there is lots of "freedom" or direction for the lines. When the movement is finally happening at a harmonic level, the lines follow the more obligatory directions: bar 3 to 4 treble Bb C Bb Ab descending through dominant to mediant, bars 3 to 4 the bass E♮ up to F ascending to the tonic.

...from Bach's BWV 851, there is a nice example of a cross relationship with degree ^6. The middle of bar 13 moves through a dominant in a deceptive progression. The harmonic rhythm is fast so each tone moving to and from ^6 is real harmonic motion and all the expected directional tendencies are fulfilled. ^6 is natural as it ascends to a dominant harmony which in turn ascends to the tonic for form the deceptive progression. At the same time the bass from from the dominant to the ^6 degree, and because the ^6 at that point in the bass is not moving to a dominant or leading tone - but rather away - it is not raised. A beat later, in the treble, the ^6 appears again but as part of a subdominant harmony, it is not raised but is part of an auxilliary motion around ^5.

Notice the difference in treatment of raised ^6 & ^7 in the ascent here B♮ C♯ D where each tone is part of harmonic movements dominant to tonic, versus the example in the two-part invention where the movement is decorative within a single dominant chord and the line freely ascends and descends.

This the systematic treatment of ^6 & ^7 in minor. The treatment is strongly directional when occurring at the harmonic level, but ascends/descends freely on the decorative/diminution level.