Figure 2 shows the orbifold T2
/S2, the space of unordered pairs of pitch classes. It
is a Möbius strip, a square whose left edge is given a half twist and identified with its right.
The orbifold is singular at its top and bottom edges, which act like mirrors (18). Any
4
bijective voice leading between pairs of pitches or pairs of pitch classes can be associated
with a path on Fig. 2 (Movie S2). Measures of voice-leading size determine these paths’
lengths. They are the images of line segments in the parent spaces Tn
and Rn
, and are
either line segments in the orbifold, or “reflected” line segments that bounce off its singular
edges. For example, the voice leading (C, Df)(Df, C) reflects off the orbifold’s upper
mirror boundary (Fig. 2).