Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In mathematics, a dual graph of a given planar graph G is a graph which has a vertex for each plane region of G, and an edge for each edge in G joining two neighboring regions, for a certain embedding of G. The term "dual" is used because this property is symmetric, meaning that if H is a dual of G, then G is a dual of H (if G is connected). The same notation of duality may also be used for more general embeddings of graphs on manifolds.