IX. Structural Terminology
Stack
A chord in the Summit orbit — the quintal reference sonority. “The C stack” = the [7,7,7] chord rooted on C = {C, G, D, A}.
Axis Pair
Two Summit chords separated by a tritone (T₆), sharing the same Saddle chord(s). The OTH analogue of tritone-related keys. Example: the C stack {C,G,D,A} and the F♯ stack {F♯,C♯,G♯,D♯}.
Key Region
The set of chords in B gravitationally organized around a particular Summit chord. The 12 Summit chords define 12 key regions, analogous to the 12 major keys.
Degree Landscape
The assignment of degree values (4, 5, 6, or 8) to every chord in B, visualized as a terrain map. The landscape has peaks (degree 8), slopes (degree 5–6), and valleys (degree 4). The overall structure is a ridge of degree-8 chords with two wings descending on either side.