Recent research on bidirectional heuristic search (BiHS) is based on the must-expand pairs theory (MEP theory), which describes which pairs of nodes must be expanded during the search to guarantee the optimality of solutions. A separate line of research in BiHS has proposed algorithms that use lower bounds that are derived from consistent heuristics during search. This paper links these two directions, providing a comprehensive unifying view and showing that both existing and novel algorithms can be derived from the MEP theory. An exhaustive set of all possible bounds is formulated, showing that any other bound will be dominated by the bounds in this set. Finally, the bounds are empirically evaluated by their individual and joint contribution to the efficiency of the search.