Given a planar graph 
, its geometric dual 
is constructed by placing a vertex in each region of 
(including the exterior region) and,
if two regions 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 showed that the geometric dual graph and combinatorial dual graph are equivalent (Harary 1994, p. 115), and so may simply be called "the" dual graph.
