This is my big new thing, since several months now when I discovered Harry Partch and John Schneider. There are apparently two ways to go here. One can base scales on the natural harmonics of a vibrating string or column of air, the harmonic series resulting in unequal intervals within the octave, or else simply divide the octave into an arbitrary number of equal divisions, conventionally twelve, called equal temperament.

I will make some notes here for follow-up reading. The excellent, very readable book by Ross W Duffin ‘Hoe Equal Temperament Ruined Harmony (and why you should care’ has put me onto Easley Blackwood:

From Wikipedia, the free encyclopedia
Easley Blackwood, (born April 21, 1933), is a professor of music, a concert pianist, a composer of music, some using unusual tunings, and the author of books on music theory, including his research into the properties of microtonal tunings and traditional harmony.