  Low-energy meson-meson scattering data are a powerful testing ground for
quark models. Here, we describe the behaviour at threshold of S-wave
scattering-matrix singularities.
  The majority of the full scattering-matrix mesonic poles stem from an
underlying confinement spectrum. However, the light scalar mesons K0*(830),
a0(980), f0(400-1200), and f0(980) do not, but instead originate in 3P0-barrier
semi-bound states. We show that the behaviour of the corresponding poles is
identical at threshold.
  In passing, the light-meson sector is given a firm basis.
