Conjugate with the enthalpy, with these arguments, the other characteristic function of state of a thermodynamic system is its entropy, as a function, S [ p ]( H, p, {N i }) , of the same list of variables of state, except that the entropy, S [ p ], is replaced in the list by the enthalpy, H. It expresses the entropy representation. The state variables H, p, and { N i } are said to be the natural state variables in this representation. They are suitable for describing processes in which they are experimentally controlled. For example, H and p can be controlled by allowing heat transfer, and by varying only the external pressure on the piston that sets the volume of the system.