1.2. KNOTTE D SURFACE DIAGRAMS
7. A diagram of a surface that has triple points
Figure 7 illustrates a broken surface diagram of a properly embedded disk that
came from a surface with triple points. To obtain the diagram of a sphere, double
this disk along its boundary. We will see in Section 4.2 that the existence of triple
points can cause a given surface in R 3 to not have any broken surface diagrams.