What the book refers to as “modal exchange” is sometimes referred to as modal mixture, or borrowed chords. The harmonic palette is expanded through the use of scale degrees and harmonies from the parallel major or minor mode.
Most modal exchange involves bringing harmonies from the parallel minor into the major mode. The lowered scale degrees are flat-^3, flat-^6, and flat-^7, resulting in the following chords:
- i (minor tonic, utilizing flat-^3): mainly an embellishing or transient harmony, not as the goal of a cadence.
- ii° and iv (diminished supertonic and half-diminished supertonic seventh, minor subdominant, both utilizing flat-^6): as predominant harmonies, embellishing harmonies (I – iv64 – I), or as embellishments of their major-mode counterparts. Note that the scale degree flat-^6 will almost always move to the fifth scale degree (^5).
- bVI (flat submediant, involving flat-^6 and flat-^3): as part of a deceptive cadence or harmonic progression from V. It’s less likely to move directly to the dominant (but still can).
- bIII and bVII are used less frequently (bVII least of all).
Note that the case of the Roman numeral tells us the quality of the harmony. If the root of the harmony is altered from the current mode, then this must also be indicated (bVI is built on the flat-^6 scale degree in major).
For part-writing purposes, flats tend to move downward to the next harmony. Flat-^6 almost always goes to ^5. Be particularly careful that you don’t create augmented melodic (linear) intervals through the use of modal exchange.
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