The term bidirected edge has two related meanings. In a directed graph, if both 
and 
are edges, the pair is sometimes called a bidirected edge (van Dam and Omidi 2018).
More generally, in a bidirected graph, each end of a single edge is assigned one of two orientations independently. Thus, for an edge with distinct endpoints 
and 
, there are four types: the two ordinary directed
edges 
and 
,
an edge with an arrowhead at each endpoint, and an edge with an arrow tail at each
endpoint. An undirected edge, which has no endpoint
orientations, is not one of the four types (Bessouf et al. 2019, p. 296).
