  By studying the spin structure of an S-wave meson-antimeson system in the
heavy quark limit, we find two selection rules for the $c\bar{c}$ spin
$J_{c\bar{c}}$ being only 1: (a) $J=min(J_\ell)-1$ or $J=max(J_\ell)+1$; (b)
$J^C=1^+,2^-,3^+,\cdots$ if the two mesons are different but belong to the same
doublet. Here $J_\ell$ is the total angular momentum of the light degree of
freedom. The rules may constrain decay channels of a meson-antimeson molecule
or resonance. For the recently observed $Z_c(3900)$ state, it in principle can
decay into $J_{c\bar{c}}=0$ charmonium channels if it is a $D\bar{D}^*$
molecule or resonance. The non-observation of $h_c\pi$ decay might imply that
this state is a tetraquark.
