Well, maybe it's about four months too late to reference Jimmy Webb's iconic book, Tunesmith,
but in the theory-laden "middle eight" of that tome he does indeed discuss the many ways in which a melody can be "harmonized" ... and why.
Very interesting reading from the scholar(!) who introduced us all to the lineman from the county ...
"Three chords and the Truth."
I finally got both of my copies signed – my hardback library copy and my now-well-thumbed softcover reading copy. When he signed that
one, he commented, "this is a well-used book!"
To which I promptly replied, "yes sir, that it is!" He is such
a gentleman ...
Yes, that's always acceptable, but in the same way that vanilla ice cream is always acceptable if you've never tasted Ben & Jerry's.® But this would never have led you to harmonize Stairway to Heaven
in a way so distinctive that it led to a lawsuit ... over what turned out to be a University lesson.
Two of the core concepts in Professor
Webb's could-be college textbook are "the principle of substitution" and the importance of "chord inversions." Any chord can be substituted
for any other as long as it shares at least one
note with the original. (Second- and third-generation "derivative substitutions" can be made as well, growing increasingly more exotic.) A simple triad of course has three notes, but sus2, sus4, +6, +7, +9 and so-on add more possibilities. A chord can also be inverted
– any chord has one fewer inversion than it has notes – by moving the bottom note to the top. This "opens up" the chord: a minor-third, when inverted, always becomes a major-sixth. A "suspension" that resolved down now resolves up, and vice-versa. Chord progressions can both accompany
a melody, and contain
a possible melody. He systematically shows how to explore
the possibilities, and just how many possibilities there are to explore.
As you can readily guess, I recommend this book to everyone. The "middle eight" is very
intense, difficult reading – and worth it. Have a keyboard nearby.