  In this note we first introduce the theoretical formalism of the
unitarization of the two-body scattering both in the infinite and finite
volumes. Then we apply this formalism to the study of the $Z_c(3900)$ resonance
in the $J/\psi\,\pi$ and $D\bar{D}^{*}$ coupled-channel scattering. The recent
lattice finite-volume spectra are confronted with our predictions.
