5.3.1 Torus

If x lies on an edge, then each of the two points in [x] has a half-disc-like neighbourhood. When we identify edges, these neighbourhoods fit together to form disc-like neighbourhoods on the torus, as Figure 105 shows.

Figure 105

If x is a vertex, then each of the four points in [x] has a half-disc-like neighbourhood (in the shape of a quarter-disc). When we identify edges, these neighbourhoods fit together to form a disc-like neighbourhood on the torus, as Figure 106 shows.

Figure 106