  This work critics on nature of thermodynamics coordinates and on roles of the
variables in the equation of state (EoS). Coordinate variables in the EoS are
analyzed so that central concepts are noticed and are used to lay a foundation
in building of a new EoS or in testing EoS status of a newly constructed
empirical equation. With these concepts, we classify EoS into two classes. We
find that the EoS of market with unitary price demand and linear
price-dependent supply function proposed by \cite{GumjMarket}, is not an EoS
because it has only one degree of freedom.
