Ethan Hein on Prince's guitar solo for "Kiss", some scattered thoughts on modal mutation across asymmetrical and symmetrical modes as an expressive option

http://www.ethanhein.com/wp/2021/the-best-guitar-solo-ever-recorded/
Now I tend to shy away from GOAT (greatest of all time) declarations in general but Prince's solo on "Kiss" is, to my mind and ears, indisputably one of the top tier guitar solos in rock and pop.  He discusses the solo at some length in the post linked above.

Coming at things more from the background of a classical guitarist with some choral background the thing that leaps out about the start of the solo is the A, C, G chord.  I wouldn't classify it as A major or A minor 7 in itself because the song often sounds like it's in A major (per the thirds added by the vocal line during the verses.  But since I've been a Ben Johnston fan for a few years what kind of third is bluesy, so we could say there could be a minor third, a major third, or a "neutral" third that's not quite either of those notes.  

All of that is preliminary to the entrance of the chord in the guitar solo.  That oscillation between the G natural and the F# in the chordal solo can imply D7 chord shapes (per Ethan Hein's commentary) but what makes the Am7-D7 alternation in the chord shape fun is not that the D is anywhere in the harmony, it's that the solo introduces a dorian modality that disrupts the A major feel the song has had up to the entrance of the guitar solo.  Dorian, for music theorists, is a symmetrical scale, it's the same going up and down or, rather, if you "invert" the mode from the root note you won't get a change.  If you "invert" lydian's intervals and start from the same root note you'll get locrian. I got into that a little with the post "the diatonic modes as inversions of each other, a little riff on George Rochberg's proposal that Western music has had cycles of favoring symmetrical and assymetrical paradigms for pitch organization in the last thousand years" 

To put this another way, that opening alternation Prince uses between the Am7 shape and the D7 shape is explicable as a variation on the "So What" riff that Ethan Hein has discussed elsewhere at his blog.  I'm not saying Prince was consciously copying the "So What" riff so much as I'm pointing out that the "So What" riff has so saturated jazz, funk and rock that variations of that IV-i7 oscillation show up all sorts of places.

The symmetricality of dorian is cool if you don't use it as a musical crutch, which is why I had a response post to Hein's post on "So What" pointing out that Davis' neapolitan chord substitution in "So What" was what saved it from being a cliche i7-IV-i7 groove that can show up in post-psychedelic rock.  Zappa's "Nine Types of Industrial Polluion" and "King Kong" are more conventional uses of the dorian mode in rock, for instance, and I like both of those but they are examples of what is, I'd say, the more conventional use of dorian.

As Rochberg proposed in his later writings music history in the Western world in the last thousand or so years has alternated between cycles of favoring symmetrical and asymmetrical forms of pitch organization, probably most easily described as a contrast between the symmetrical dorian mode (plainchant, etc) and the major/minor key system of the common practice ear (18th through 19th centuries), and the emergence of chromaticism as a new variant of favoring symmetrical pitch systems.  Rochberg proposed in his later writings that rather than have cycles of favoring one or the other form of pitch organization there might be a new way to organize sound in music that could let composers and musicians fluently shift from assymetrical to symmetrical pitch systems and back within a musical work so that both approaches can co-exist.

I trust that you have inferred where I'm going with this regarding jazz and blues. :)  Flexible modality has been favored in all kinds of music in the last two centuries whether we're looking at chromaticism in Chopin, Wagner and Lizst or whether we're looking at microtonal elements in blues and post-Partsch post-common practice music.  That Anton Reicha recommended quarter-tones for opera singers as an expressive option back around 1810 lets us know that even if the Bohemian composer wouldn't have recognized those quarter-tones as the blue notes we hear in blues he was aware that such microtonal deviations from notated pitch existed; could be used for expressive purposes; and recommended their use.

I don't have it on me but in Defining Russia Musically Richard Taruskin had a chapter called "Scriabin and the Superhuman" where he discussed how Scriabin's mysticism resembled that of Balzac and how in the post-Wagnerian theosophical scenes there was an aspiration toward "double gender", a vatic androgynous conception of humanity that  transended male and female and that, in musical terms, was attempted as a depictional process by using chromaticism and symmetrical approaches to pitch organization (see George Rochberg's works such as A Dance of Polar Opposites for more abstruse ruminations on the pitch organization stuff without the mysticism as such).  

I'm not really a big fan of Scriabin, actually, but I am proposing that when people like Scriabin aspired to a hyperchromatic post-tonal way to organize music that went beyond the tuning systems and diatonic scales of their era (and a Scriabin disciple named Ivan Wyschnegradsky ran with composing in quarter-tones, for folks who want to explore hyper-chromatic and microtonal music in the Russian and related traditions) they were consciously aspiring to making music by a set of ideals that was evolving on the other side of the Atlantic in the form of blues and jazz.  I'm spitballing a bit here on a weekend but I wonder if that helps to explain why fusions of jazz and classical have happened a la Nikolai Kapustin and some other composers whereas in the West jazz/classical fusions have had a rougher time of it because of, well, racial purity politics across all the color and ethnic lines.  There have been musicians who decide to just ignore those kinds of purity codes about how to organize music and run with what they find fun.  I think you'd be better off listening to Prince than Scriabin, personally, but I don't want to tell Scriabin fans they can't enjoy his music.  

The single most important thing about music theory that you may never have been told in a music theory course is that all music theory is post hoc. Theorists formulate observations about what it is that "works" about the music they and other people love and then work their way back to theories that explain what works.  You may have run into teachers who treat music theory as prescriptive rather than descriptive but descriptive is what theory is.  All those voice-leading rules that you may have found annoying in a theory class if you play instruments make a completely different kind of sense if you're a choral singer who has sung unaccompanied choral music based on Latin language liturgical musical practices. By contrast, if you're more steeped in Balkan and Baltic traditional music the prospect of parallel minor sevenths sung by a choir isn't so alien to you, and some Pacific Northwest Native American songs have parallel seconds and occasional tritones.  

What can make many a conservative writer on music theory aggravating to me, even as I'm someone who's moderately conservative, is that a whole lot of goal posts get moved depending on who the target of complaint is.  Pop and rock get a drubbing for not following "the rules" of 18th century Baroque era counterpoint but that can often be a Baroque era counterpoint as imagined by 19th century theorists and pedagogues. Then the Romantic heroes of music are praised for not feeling beholden to "the rules" of 18th century practice.  Haydn didn't feel obliged to avoid "doubled octaves" when he would assign the same melody to oboes and bassoons in a symphony because even if northern German music critics thought that was "bad voice leading" Haydn could hear that the two double reed instruments would reinforce a musical line across two registers with a unified sound that he wanted heard.  I.e. Haydn was writing to please audiences not theorists. He could follow "the rules" when he felt like it but as theory books on form so often concede, you'll read about how the "textbook" way to write in an 18th century style is to do X, Y and Z with the caveat that "keep in mind that you may never see Haydn do any of this stuff".  As Richard Taruskin has cheerfully pointed out, the Haydn whom Romantics presented as the stuffy rule-maker never bothered to follow any of the rules they retroactively imputed to him!

On the blues side there are those who say that blues is a 12-bar formula.  It's not. It's a flexible musical vocabulary that gives you room to do all kinds of things and you "can" play 12'bar blues if you want but you don't have to.  If you want to repeat the V-IV more than once before you get to the turnaround that goes back to the I you have the option to do that.  If you had any doubts about that go listen to "Kiss" by Prince and he'll show you you've got more options than playing 12-bar blues as described in a theory course. 

POSTSCRIPT

I think Susan McLary has a discussion of "Kiss" in her book Conventional Wisdom.