An undirected edge joining graph vertices 
and 
does not distinguish an initial endpoint from a terminal endpoint.
In a simple graph, the edge joining distinct vertices
and 
is the unordered pair 
, commonly abbreviated 
, and is drawn without an arrowhead (Harary 1994).
In the Wolfram Language, an undirected edge is represented by UndirectedEdge[u, v]. Inside a graph constructor, the two-way rule u <-> v is converted to the same form.
