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In some books one encounters statements about modal mixture which, in my view, reveal misunderstanding of how major and minor modes influence each other. For example, on p. 279 of Concise Introduction to Tonal Harmony by Burstein/Straus, one reads: "Since degrees 6 and 7 are part of the ascending melodic minor scale, IV6 and V6 are not borrowed chords".
According to this logic, minor IV is a borrowed chord in major, but major IV is not a borrowed chord in minor. This is another way of saying: "The minor scale has harmonic and melodic versions, but the major scale does not."
I will try to clear the above discrepancies in the following manner.
At the bottom of modal mixture lies the understanding that scale degrees 6 and 7 fluctuate in both major and minor scales, and this fluctuation reflects the influence of the opposite mode. Scale degree 3 may also fluctuate and reveal the influence of the opposite mode, as related to the appearance of bIII and bVI in major and #IIIm and #VIm in minor. Yes, the raised mediants in minor are great borrowed chords, aren't they? Their employment is pertinent to a later style, related to 20th century tonal music.
Thus one can rightfully assume that all chords from minor may be used in major and vice versa. In some books you will see a presentation of a "major-minor key" whose tonic is major but all other chords are borrowed from minor, and next to it – a presentation of a "minor-major key" whose tonic is minor but all other chords are borrowed from major. Whichever tonic has the control, it determines the primary (controlling) modality.
The core of tonality is the pure diatonic system which is reflected by the key signature of any major or minor key. The harmonic and melodic scales of major and minor expand the boundaries of tonality by reflecting the influence of the opposite mode. Thus, harmonic minor borrows the leading tone from major to obtain more active dominant chords, and harmonic major borrows the lowered 6th degree from minor to obtain more active sudbominant chords. Notice that the upper tetrachords of these two modes are identical. Notice also that, 120 years ago, N.R. Korsakov based his Practical Manual of Harmony on these four modes (I strongly recommend it; it has more practical information on harmony than several contemporary thick volumes put together).
Further modal mixture involves the creation of melodic minor whose upper tetrachord is entirely borrowed from the natural major scale. Melodic major, on the other hand, borrows its upper tetrachord from natural minor. In other words, when natural minor and natural major swap their upper tetrachords, the resulting chromatic scales are called "melodic minor" and "melodic major". (Of course, one should not confuse genuine melodic minor with the hybrid scale which ascends melodically and descends naturally, but that is another topic).
In addition the chords which could be considered directly borrowed from the opposite mode, there are some unique chords in the harmonic and melodic scales which are truly chromatic and do not exist in the natural scales. These are the augmented triad, the dim.7, the mM7 and the augM7. These chords are also obtained via modal mixture but only in terms of borrowed scale degrees, not as entities.
Modal mixture does not stop there. It involves borrowing chords from old modes (Lydian, Phrygian, etc.). A conspicuous result of this interaction is the Neapolitan chord, called by some theorists "Phrygian SII chord" This is why N must not be separated from the study of modal mixture. Textbook titles should not read: "Modal Mixture and the Neapolitan" (which reveals unawareness that N is a form of modal mixture) but just "Modal Mixture" with sub-chapters about chords borrowed from the opposite mode and about other chords, borrowed from the old modes.
Finally, whether you say: "major V in a minor key is obtained in harmonic minor" or "major V in a minor key is borrowed from the opposite major mode" – you are saying the same thing.
It must be remembered that the harmonic and melodic major and minor scales are forms of modal mixture.
Returning to the astonishing claim pasted in the beginning of my message, allow me to fix it this way: "Since degrees 6 and 7 fluctuate in both major and minor scales, the S and D chords which reflect this fluctuation are borrowed chords."
Thank you for your attention.
Dr. Dimitar Ninov
Texas State University
San Marcos, Texas