where for every system with degree of freedom r one may choose r variables, e.g. ( X 1 , … … , X i , Y i + 1 , … … Y r ) , 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, Ψ, is then called the Massieu function.