Thanks for following along and getting involved Jason.
What a question!
Always - if it sounds good it is good.
Any borrowed chord is making an alteration in the harmonic structure.
Above that, of course, can be melodic structure.
The borrowing we have seen in this study has been exclusively parallel major and minor key mixture. One example of the wider concept of modal mixture. Major is Ionian is a mode. Minor is Aeolian is a mode. Borrowing chords is also called modal mixture. You can have a major key progression that borrows from, say the parallel Dorian mode too.
Whichever modes are used for borrowing, because they are parallel, they will share some common tones.
If we use example 10 in E minor as an example.
E minor scale:
E, F#, G, A, B, C, D
E major scale:
E F#, G#, A, B, C#, D#
There are four common tones. If the overall harmonic structure is E minor with occasional chords borrowed from E major, then a melody could be played over the progression such that it comprised some or all of the notes from E minor except when the borrowed chords arrive. The borrowed chords coming from E major will signal the need to approach the melody differently.
The notes G, C and D are those to notice and possibly avoid / replace, depending on which chord is borrowed. They may be replaced by their alphabetical counterparts G#, C# and D# if these are chord tones within the borrowed chords. Think of the notes within the triads to know which notes in E minor are going to clash badly with chords from E major. So the melody has to be moulded and shaped to be pleasing, not dissonant.
In Example 10 A major and B major were borrowed.
A major = A, C#, E
When this chord comes along you will be targeting the note C# which comes from the E major scale.
B major = B, D#, F#
When this chord comes along you will be targeting the note D#.
If creating a vocal melody you may wish to be able to use the full palette of seven notes scales, no matter what chords are being played.
If riffing and improvising on a guitar, limiting the play to pentatonics can remove some of the difficulties though there are still notes in each that cause the same issues as above.
E minor pentatonic:
E, G, A, B, D
E major pentatonic:
E, F#, G#, B, C#
One more aspect worth mentioning here. The very common use of a major for the dominant in a minor key (most often V7) is such a desirable example of a borrowed chord that its usage led to the creation of two new scales. The reason it is so desirable and used so much is the strong and satisfying resolution from V7 to i, dominant to tonic. This occurs because the major 3rd in a V7 moving to a tonic minor chord is a leading tone, just a semitone away from the root note of the tonic. It does not appear in the minor scale.
When I presented it in example 9 above i did so as though it was just another, regular major / minor borrowing.
I deliberately chose not to mention that a chord progression in a minor key with a V7 chord is not usually spoken of as chord borrowing. The progression would more normally be described as in the harmonic minor. This is a (natural) minor with a raised 7th degree - the one that creates the leading tone. It was created specifically to give this dominant to tonic resolution in minor keys.
The chords derived from harmonic minor had an implication for melodies too and that in turn led to the creation of the melodic minor scale. It is two scales in one depending on whether ascending or descending. It navigates round the difficulty of a large interval created when raising the 7th for the harmonic minor.
That is probably enough on that subject within this topic.
I hope some or all of that makes sense.
I may need to re-read this as it is late Sunday night to be diving to these depths haha.
