  Deciding whether there is a single tree -a supertree- that summarizes the
evolutionary information in a collection of unrooted trees is a fundamental
problem in phylogenetics. We consider two versions of this question: agreement
and compatibility. In the first, the supertree is required to reflect precisely
the relationships among the species exhibited by the input trees. In the
second, the supertree can be more refined than the input trees.
  Tree compatibility can be characterized in terms of the existence of a
specific kind of triangulation in a structure known as the display graph.
Alternatively, it can be characterized as a chordal graph sandwich problem in a
structure known as the edge label intersection graph. Here, we show that the
latter characterization yields a natural characterization of compatibility in
terms of minimal cuts in the display graph, which is closely related to
compatibility of splits. We then derive a characterization for agreement.
