The internal energy, , expresses the thermodynamics of a system in the "energy-language", or in the "energy representation". As a function of state, its arguments are exclusively extensive variables of state. Alongside the internal energy, the other cardinal function of state of a thermodynamic system is its entropy, as a function, , of the same list of extensive variables of state, except that the entropy, , is replaced in the list by the internal energy, . It expresses the "entropy representation".