  We present a plausible mechanism for the origin of the $XYZ$ mesons in the
heavy meson spectra within a standard quark-model picture. We discuss the
conditions required for the existence of four--quark bound states or resonances
contributing to the heavy meson spectra, being either compact or molecular. We
concentrate on charmonium and bottomonium spectra, where several new states,
difficult to understand as simple quark-antiquark pairs, have been reported by
different experimental collaborations. The pivotal role played by entangled
meson-meson thresholds is emphasized.
