  We introduce the hypothesis that diquarks and antidiquarks in tetraquarks are
separated by a potential barrier. We show that this notion can answer
satisfactorily long standing questions challenging the diquark-antidiquark
model of exotic resonances. The tetraquark description of X and Z resonances is
shown to be compatible with present limits on the non-observation of charged
partners X^+-, of the X(3872) and the absence of a hyperfine splitting between
two different neutral states. In the same picture, Z_c and Z_b particles are
expected to form complete isospin triplets plus singlets. It is also explained
why the decay rate into final states including quarkonia are suppressed with
respect to those having open charm/beauty states.
