Why do classical teachers often tell students to play melodic minor descending as natural minor? I read that Schoenberg said it was to release tension of the leading tone but if so why don’t we do this to all scales with leading tone
Common Practice harmony is all about dominant - tonic relationships. About G7 going to C major. It NEEDS the dominant chord to contain the leading note, the seventh degree of the scale a half-step below the tonic so that it can combine with the fourth degree of the scale to make the tritone interval that powers the dominant 7th chord in wanting to resolve to the tonic. Hence the 'sharpened 7th' in the Harmonic Minor scale.
But although Common Practice loves the augmented 4th of the tritone, it complains that the augmented 2nd between the 6th and 7th degrees of the harmonic minor scale can sound a bit - well, un-melodic. Hence the Melodic Minor scale. Going up, it retains that harmonically-vital leading note, leading up to the tonic but sacrifices the 'minor-ness' of the 6th degree. Coming down, when the leading note is, by definition, NOT rising to the tonic, they can both be flattened for full minor effect.
Jazz players do it too :-)
Interesting thought about, say, major scales having a m7 when descending!
The reason may well be that if that was used, it would tend to put the sonority into the key a 4th above - original key C, with B♭ sounding, makes it sound like key F. Particularly when the tritone of E and B♭ sound together. That's the pull towards an F chord, thus a modulation. Which does actually happen quite a bit. But if it was used regularly in scales, it would stray from the diatonics that are the mainstay of Western music.
Having said that, more and more, we hear the ♭7 chord (here B♭) being used in modern music. Time for a change?
The other part of the question has been admirably solved in Laurence's answer.
Why do classical teachers often tell students to play melodic minor descending as natural minor?
It's a textbook definition. You will be tested to give back that answer.
It doesn't apply to real music where the raised
^7 degree are used in descending lines if the surrounding harmony is dominant.
I read that Schoenberg said it was to release tension of the leading tone but if so why don’t we do this to all scales with leading tone
It does happen.
But it depends on how you interpret "release tension of the leading tone" and whether you think various movements should be categorically grouped together.
Let's accept the "tension of the leading tone" to mean its tendency to ascend to the tonic.
In minor that tendency to ascend can be weakened by lowering the leading tone facilitating a descending line, like a descending bass in
i v6 iv6 V note that
v6 is a minor chord.
In major that ascending tendency can also be weakened to allow a descending line, but instead of descending to the dominant, it would be common to descend to the subdominant like
I V7/IV IV, or to put the changing
^7 degree in the bass,
I V42 IV6.
Some might call this a modulation (but that would really depend on other factors not conveyed by chord symbols alone) and this is why I said it depends on how you interpret "release tension of the leading tone." Certainly the tension of the leading tone relative to
I is released when tonicizing
IV, but you could also say the tension is being re-positioned to degree
^3 which then is the leading tone to
With letter names:
C C7 F the leading tone to
B natural, at
C7 that tone becomes
Bb - the tension has been released from
B by making it
Bb - relative to
Bb acts like the
^4 scale degree, and
E which was the
^3 degree relative to
C is now the leading tone relative to
F. So the
B tension was release, but redefining
F as a temporary tonic imbued
E with new tension as a leading tone.
So, relaxing the leading tone does facilitate moves to different chords in both minor and major keys.
I find a lot of these sort of, should we, should we not, all comes down to personal preference and context. If you blowing a solo over some changes, there's not a lot of difference when the tempo is 160 and there's a full band behind you. On the other hand, if it's a subtle solo piano composition, well then there might be a time where that sort of idea is appropriate.
I am not saying one is right or wrong, but context matters.
The melodic minor scale is a combination of parts of the other minor scales. The altered 6 and 7 are an attempt to solve the following problems that exist in the minor scale:
- The dominant often resolves to the tonic
- The dominant chord uses a #7-1 motion for a perfect cadence
- The motion between 6 and #7 is an augmented 2nd
The solution to both of these problems, is the melodic minor scale going up, which contains #6 and #7.
Regarding the scale going down, any stepwise motion from 1-7-6 does not need a #7 or a #6, since there is no leading tone motion (7-1). So, the downward version of the melodic minor scale can use, and does use, the natural minor.
[Note: the natural minor exists within the Major scale, and is closer to the natural key]
We do not do this for all scales, since the most frequently used scales are the Major, minor, and (strictly) pentatonic scales. The Major does not require any alteration, and the pentatonic scales are strictly the five notes. The minor scale is the only one of these which presents these problems in voice leading.