The Massieu function, \(\Psi\), is defined as: \(\Psi = \Psi (X_1, \dots , X_i, Y_{i+1}, \dots , Y_r )\), where for every system with degree of freedom \(r\) one may choose \(r\) variables, e.g. , to define a coordinate system, where \(X\) and \(Y\) are extensive and intensive variables, respectively, and where at least one extensive variable must be within this set in order to define the size of the system. The \((r + 1)^{th}\) variable,\(\Psi\) , is then called the Massieu function.