The pillow is a special kind of orbifold. It is not just locally a quotient by a fi
nite group action; we will show that it is globally the quotient of the torus by an
action of the 2-element group Z2. First recall that the torus can be regarded as the quotient space of the plane JR2 under the action of the group ?} by translation:
(i, j) : (x, y) --+ (x + i, y +})[54]. In this way, one can play chess on the torus
by playing on the infinite plane and identifying appropriate squares; if the plane is
tiled by unit-square chessboards, centered on the vertices of the integer lattice ?}