Figure 4 shows all ways of placing anchors {u,v} on the quartet tree a b.c d. Anchors can be sisters, placed on the internal branch (Case 1) or on a tip branch (Case 2; w.l.o.g, we pick the branch pending to d). When anchors are not sisters, they can be both placed on the internal branch (Case 3), or one on the internal branch and the other on a tip branch (Case 4), or they can be both on terminal branches, which can be done in three ways: u and v can be on the same terminal branch (Case 5), on different but adjacent branches (Case 6), or on non-adjacent branches (Case 7).