  We review selected lattice results on the charmonium spectrum and first
attempts to search for the existence of exotic states. The hadro-quarkonium
model was proposed to interpret some of the exotic states as a quarkonium core
inside a hadron. We present a lattice study of the hadro-quarkonium model in
the limit of static quarks. The charm quark decouples in low energy observables
and binding energies of charmonium. In a model calculation we are able to
evaluate the corrections to decoupling of the charm quark in the continuum.
