  A menage a trois is very different from an ordinary family. Similarly, exotic
hadrons with both qq and qbar q pairs have important color-space correlations
that are completely absent in ordinary mesons and baryons. The presence of both
types of pairs requires attention to the basic QCD physics that the q qbar
interaction is much stronger than the qq interaction. This new physics in
multiquark systems produces color structures totally different from those of
normal hadrons, for example the ud system is utterly unlike the ud diquark in
the uds Lambda baryon. The color-space correlations produce unusual
experimental properties in tetraquarks with heavy quark pairs which may be
relevant for newly discovered mesons like the X(3872) resonance. Tetraquark
masses can be below the two-meson threshold for sufficiently high quark masses.
A simple model calculation shows the b q bbar ubar and b q cbar qbar
tetraquarks below the B Bbar and B Dbar thresholds. Some of these states have
exotic electric charge and their decays might have striking signatures
involving monoenergetic photons and/or pions.
