  As a first step, a simple and pedagogical recall of the $\eta$-$\eta'$ system
is presented, in which the role of the axial anomaly, related to the
heterochiral nature of the multiplet of (pseudo)scalar states, is underlined.
As a consequence, $\eta$ is close to the octet and $\eta'$ to the singlet
configuration. On the contrary, for vector and tensor states, which belong to
homochiral multiplets, no anomalous contribution to masses and mixing is
present. Then, the isoscalar physical states are to a very good approximation
nonstrange and strange, respectively. Finally, for pseudotensor states, which
are part of an heterochiral multiplet (just as pseudoscalar ones), a sizable
anomalous term is expected: $\eta_2 (1645)$ roughly corresponds to the octet
and $\eta_2 (1870)$ to the singlet.
