Each map in the plane can be represented by a graph. To set up this correspondence, each region of the map is represented by a vertex. Edges connect to vertices if the regions represented by these vertices have a common border. Two regions that touch at only one point are not considered adjacent. The resulting graph is called the dual graph of the map. By the way in which dual graphs of maps are constructed, it is clear that any map in the plane has a planar dual graph. Figure 2 displays the dual graphs that correspond to the map shown in Figure 1.