The theoretical and psychological distances among keys can be represented by the "circle of fifths" and on a rectangular sheet.

Music Theory Cognitive Psychology

from Krumhansl & Kessler, 1982

Note that the left and right sides of the rectangle are the same, as are the top and bottom. This means that the sheet can be folded to form a torus:

from Krumhansl (1990)

The torus is a representation of harmonic space. A piece of music moves around in this space.

To see the activation patterns of musical stimuli on the toroidal surface, click here.