  Nowadays, there exists an abundance of theoretical approaches towards the
mesonic spectrum, ranging from confinement models of all kinds, i.e.,
glueballs, and quark-antiquark, multiquark and hybrid configurations, to models
in which only mesonic degrees of freedom are taken into account. Nature seems
to come out somewhere in the middle, neither preferring pure bound states, nor
effective meson-meson physics with only coupling constants and possibly form
factors. As a matter of fact, apart from a few exceptions, like pions and
kaons, Nature does not allow us to study mesonic bound states of any kind,
which is equivalent to saying that such states do not really exist. Hence,
instead of extrapolating from pions and kaons to the remainder of the meson
family, it is more democratic to consider pions and kaons mesonic resonances
that happen to come out below the lowest threshold for strong decay.
Nevertheless, confinement is an important ingredient for understanding the many
regularities observed in mesonic spectra. Therefore, excluding quark degrees of
freedom is also not the most obvious way of describing mesons in general, and
scalars and axial-vectors in particular.
