  It is proposed that meson resonances are linear combinations of q-qbar and
meson-meson (MM); baryon resonances are combinations of qqq and meson-baryon
(MB). Mixing between these combinations arises via decays of confined states to
meson-meson or meson-baryon. There is a precise analogy with the covalent bond
in molecular physics; it helps to visualise what is happening physically. One
eigenstate is lowered by the mixing; the other moves up and normally increases
in width. Cusps arise at thresholds. At sharp thresholds due to S-wave
2-particle decays, these cusps play a conspicuous role in many sets of data.
The overall pattern of light mesons is consistent with nearly linear Regge
trajectories, hence q-qbar components. There is no obvious reason why this
pattern should arise from dynamically generated states without q-qbar content.
