Wikipedia on "pitch space"

Here is the link:

http://en.wikipedia.org/wiki/Pitch_space

This has some interesting points. The "regional chart" of chord
relationships, according to this, goes back to Vial in 1767. It is
closely related to the hexagonal tiling of 5-limit triads, which
itself is dual to the lattice of 5-limit pitch classes. I'm personally
fond of this chart since I deconstructed it to the tiling of chords,
and then went to the lattice of notes, getting it all from Schoenberg.
It is interesting that it has such a pedigree, going back well before
Riemann.