When I introduce the Key Circle to my students, which is usually the 1st or 2nd lesson, I say something like,
"The Key Circle is very deep and can be used for all kinds of things but at this point we're going to use it for a very simple, single thing; a mechanism to memorize note names on the 6th and 5th strings.
I recently ran across this video on John Coltrane's "Giant Steps." Even though the main subject is "Giant Steps" I thought it did a good job of communicating some of the depth of the Key Circle to those without an understanding of music theory, and how math, frequency and vibration are all part of what it illustrates.
The video speaks of the Key Circle as a cycle of 5ths; I use it as a cycle of 4ths. This is because the guitar is tuned in 4ths so the counter-clockwise movement (4ths) is more applicable to the instrument.
This is the very simple, guitar-oriented Key Circle graphic that I created for the 3rd lesson of my 5-Lesson Foundational Series:
Coltrane flipped the Circle so that the sharp keys are on the left and the flat keys are on the right. I don't know why. This is John Coltrane's diagram:
There is ample documentation that Coltrane was interested in physics and conversant in the work of Einstein. Dartmouth professor and astrophysicist Stephon Alexander says that "Giant Steps" and Coltrane's Key Circle diagram “changed my whole research direction… led to basically a discovery in physics.”
Alexander describes his jazz epiphany as occasioned by a complex diagram Coltrane gave legendary jazz musician and University of Massachusetts professor Yusef Lateef in 1967. "I thought the diagram was related to another and seemingly unrelated field of study—quantum gravity,” he writes in a Business Insider essay on his discovery, “What I had realized... was that the same geometric principle that motivated Einstein’s theory was reflected in Coltrane’s diagram.”
There are other, more guitar-centric geometrical diagrams - check out Pat Martino's Geometric Revelation - the Nature of Guitar.
Anyway, whether you understand any of this or not, check out the video, I thought it was worthwhile:
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