Register of primary diagonal for each pitch . . .

Okay, here's the main diagonal I envision for each pitch on the guitar:

A4 and A3 (both diagonals necessary)
A#4 and A#3 (ditto)
Bb#4 and Bb3
B3
C4
C#4/Db4
D4
D#4/Eb4
E4
F4
F#4/Gb4
G4
G#4/Ab4

That makes the middle C octave the octave of choice for each diagonal on the guitar, with the exception of the A#, A, Bb, and B diagonals.

That means the guitar is, primarily, a middle-range instrument. Lower sounds and higher sounds are like special effects.

An "a-ha" moment!

Yay! My work on diagonals is paying off. I saw a grand diagonal of E4s all the way across the fretboard from string 1 to string 6. That was exciting because it was a moment of complete peripheral vision: in other words, I can see where a particular scale is in every place it happens all over the fretboard in one glance. That has never happened before . . .