2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/187513The discovery in the last few years of the $X, Y$ and $Z$ states of the extended charmonium family has highlighted the importance of the closeness of decay channels to an understanding of these mesons. We aid this debate by illustrating a simple calculational procedure for including the effect of open and nearby closed channels.4 pages, 2 figures Revised version: (1) corrected 2 typos in Table II, (2) additional text in penultimate paragraph to clarify the calculation of mass-shifts for $η_c'$ and $η_c"$. We thank colleagues for pointing out confusing wording of previous textHigh Energy Physics - Phenomenologytext