We prove the conjecture of
Menasco and Zhang that a completely tubing compressible tangle consists of at most
two families of parallel strands. This conjecture is related to problems concerning
graphs in 3-manifolds, and follows from a theorem that states that a 1-vertex graph
in M is standard, in a certain sense, if and only if the exteriors of all its nontrivial
subgraphs are handlebodies.