Unsurprisingly, Gann mentions his experience and observation as a music teacher that ragtime and barbershop afford rich harmonic vocabularies in non-modulatory contexts, which makes it easier for him to introduce students to more advanced chord vocabularies within popular styles than he would if he were entirely beholden to the art music canon.
[this was back from a post Gann wrote in February 2015]... I don’t teach modulation until March, and nearly all classical music modulates, while in ragtime and barbershop I can get a dense wealth of varied harmonic functions without having to worry about changing key. Plus, classical music is so varied in its techniques that I used to spend a lot of time cherry-picking my examples to find pieces with which to teach a certain principle. Contrariwise, if I want to teach common-tone diminished sevenths, almost any ragtime will do; if I want secondary dominants around the circle of fifths, any barbershop quartet will do. If I need further examples, I just turn to the next page. When I teach pivot-chord modulation I’ll turn to Schubert and Schumann, though even there, mid-century Broadway tunes offer wonderful modulation paradigms within a three-page song, and West Side Story gives me both those and a lot of imaginative uses of the French sixth chord. Besides, the complexities of these chords from “Lida Rose” already had the students pretty distraught:
[music example, not presented here]
Brahms would have been easier. I don’t mind letting it viscerally sink in that they would need to develop a stricter range of technical skills to pursue a career in Broadway music or film scoring than to hang out a shingle as some anarchic avant-garde composer. They need some respect for the amount of expertise even a supposedly “fun” musical career will demand of them. [emphasis added]
I’m a classical music nerd, and when I was a young snob I probably bought into some foolishness about the inviolable line between high and low art. But I studied aesthetics in college, I worked as a music critic, I’ve analyzed a ton of music, and I never tripped over that line; nothing ever went “click” when I crossed it. It is, at the very least, pedagogically irrelevant. I’ll take my chords wherever I can find them rich and juicy and in vast quantities. (And still the students think I’m some highbrow classical martinet.)
Snobbery can and does exist across either side of the high and low art divides. The older I get the more implausible I find those accounts that have it that blues "broke all the rules". That's nonsense and it only makes sense even as a polemic if we're chained to a basically 19th century or early 20th century conception of music pedagogy. Blues can have the same I, IV and V chords as any string quartet by Reicha. Augmented sixth chord substitutions are possible.
When people suggest that early guitar blues somehow broke the rules that's just absurd. Anyone who has any musical competency and spends ten minutes fiddling with a guitar in an open chord tuning will appreciate that root to root harmonic shifts are limited on the guitar in open chord tunings. So what? Factor in that there's second-inversion forms of the subdominant or even first inversion forms and the harmonic changes are still audible, palpable even. Only someone who is simultaneously musically literate but has a woefully untrained ear would be unable to hear the ways that Fred McDowell or John Lee Hooker use second-inversion subdominants in their open chord tunings because that's what's available. You hear and feel the subdominant arrival even if, merely going by what's on the page or what you hear in the droning bass strings it seems as though no chord changes have taken place. That's not the case at all.
You don't have to have a trained ear to hear its effect, but you sure are helped by knowing variants of subdominant chords.
Take this ... John Lee Hooker singing "Ezekiel Saw the Wheel"
The subdominant harmony appears at 0:24 but he dropped the volume down low and played a dominant seventh chord on the subdominant harmony. But it's there. Now, sure, it might not function in precisely the same way as an 18th century sonata theme but it doesn't need to.
Just because there's inevitably going to be second inversion chord formations due to open chord tunings; just because there are pedal tones at the top and bottom strings that suggest or outline what seem to be harmonically or melodically static outer voices doesn't mean there isn't harmonic rhythm or harmonic movement going on.
Another way to put this is that if blues on the guitar were assed by guitarists rather than pianists the idea that blues somehow "broke all the rules" is easier to dispel.
If anything the blues and jazz and vernacular traditions preserved a lot of wonderful tonal and harmonic idioms from the 19th century that the highbrow art music scene was trying to divest itself of.
But I only intended to write a little bit on this topic. Gann's made the point eloquently himself but I couldn't help but add a few things to what he already wrote. Vernacular idioms like blues and ragtime can keep you busy for a lifetime even if you don't commit yourself to the theoretical and practical goal of finding ways to take those beautiful styles of music and making sonatas out of them.
But it can be done. Ragtime fusion into sonata forms took me twenty years of slow and steady experimentation to arrive at but I know it can be done. If people play bum notes in an Elliot Carter performance it's not that people won't be able to work out a bad note has been played, but I've felt for a long time that the avant garde scene has become too hermetic. It's okay to find ways to bring popular styles that are a century old into the syntactic scripting procedures colloquially and perhaps dubiously known as "classical" forms.