Blocking pair in stable matching ($r{\succ }_{\ell }M(\ell ),\ell {\succ }_{r}M(r)$)

An acceptable unused edge whose endpoints both prefer each other to their partners in a matching in a graph, treating every acceptable partner as preferable to being unmatched. A stable matching has no such edge. In the worked example, $(B,Y)$ blocks $N$ even though $A$ already has its first choice.