Given a planar graph 
, its geometric dual 
is constructed by placing a vertex in each graph
face of 
(including the exterior face) and, if two faces have an edge 
in common, joining the corresponding vertices by an edge 
crossing only 
. The result is always a planar pseudograph.
However, an abstract graph with more than one embedding on the sphere can give rise
to more than one dual.
Whitney (1932) showed that the geometric dual graph and combinatorial dual graph are equivalent for planar graphs (Fleischner 1973; Harary 1994, p. 115), and so may simply be called "the" dual graph.
