  Successful thermodynamic scaling of the structural alpha-relaxation time or
transport coefficients of glass-forming liquids determined at various
temperatures T and pressures P means the data conform to a single function of
the product variable TVgamma, where V is the specific volume and gamma is a
material specific constant. In the past two decades we have witnessed
successful TVgamma-scaling in many molecular, polymeric, and even metallic
glass-formers, and gamma is related to the slope of the repulsive part of the
intermolecular potential. The advances made indicate TVgamma-scaling is an
important aspect of the dynamic and thermodynamic properties of glass-formers.
In this paper we show the origin of TVgamma-scaling is not from the structural
alpha-relaxation time. Instead it comes from its precursor, the
Johari-Goldstein beta-relaxation or the primitive relaxation of the Coupling
Model and their relaxation times or tau_0 respectively. It is remarkable that
all relaxation times are functions of TVgamma with the same gama, as well as
the fractional exponent of the Kohlrausch correlation function of the
structural alpha-relaxation. We arrive at this conclusion convincingly based on
corroborative evidences from a number of experiments and molecular dynamics
simulations performed on a wide variety of glass-formers and in conjunction
with consistency with the predictions of the Coupling Model.
