Musically, the chroma of a note is often more important than its octave. It is
therefore useful to identify all pitches p and p + 12. The result is a circular quotient space
(pitch-class space) that mathematicians call R/12Z (Fig. S3). (For a glossary of terms and
symbols, see Table S1.) Points in this space (pitch classes) provide numerical alternatives
to the familiar letter-names of Western music theory: C = 0, Cs/Df = 1, D = 2, D quarter
tone sharp = 2.5, etc. Western music typically uses only a discrete lattice of points in this
space. Here I consider the more general, continuous case. This is because the symmetrical
chords that influence voice-leading behavior need not lie on the discrete lattice.