It is worthwhile to call attention to some important aspects of this development. The hypothetical ideal gas standard state is a wholly theoretical construct. We create this “substance” only because it is convenient to have a name for the “unreal” state of substance A, whose Gibbs free energy we have denoted as $$G^o_A\left({HIG}^o\right)={\Delta }_fG^o_A\left({HIG}^o\right)$$. We have developed procedures for calculating $${\Delta }_fG^o_A\left({HIG}^o\right)$$ from the properties of the corresponding real gas. Given the properties of real gas A, these procedures determine $${\Delta }_fG^o_A\left({HIG}^o\right)$$ uniquely. $${\Delta }_fG^o_A\left({HIG}^o\right)$$ is a useful quantity; the calculation of $${\Delta }_fG^o_A\left({HIG}^o\right)$$ is “real” even though the substance it putatively describes is not.