  We investigate the relation between the physical pion pole and screening
masses and the mesonic fluctuation scale in low-energy QCD, which relates to
the curvature of the mesonic effective potential. This relation is important
for the correct relative weight of quantum, thermal and density fluctuations.
Hence, it governs the location of phase boundaries as well as the phase
structure of QCD. The identification of the correct physics scales is also
primarily important for the correct adjustment of the parameters of effective
models for low-energy QCD. It is shown that subject to an appropriate
definition of the latter, all these scales agree at vanishing temperature,
while they deviate from each other at finite temperature.
