The thermodynamic functions "U", "S", "H", "A" and "G" are state functions. Generally, neither work nor heat is a state function. An "exact differential" is sometimes also called a 'total differential', or a 'full differential', or, in the study of differential geometry, it is termed an exact form. If three variables, formula_26, formula_27 and formula_28 are bound by the condition formula_29 for some differentiable function formula_30, then the following total differentials exist  Substituting the first equation into the second and rearranging, we obtain  Since formula_27 and formula_28 are independent variables, formula_38 and formula_39 may be chosen without restriction. For this last equation to hold in general, the bracketed terms must be equal to zero. Setting the first term in brackets equal to zero yields  A slight rearrangement gives a reciprocity relation,