 The formulas used in the axiom systems discussed so far are closed, but some free logics allow open formulas—i.e., formulas that contain free variables. These logics follow one of two conventions for variable assignments. Those that assign to each free variable a member of \(\bD\) are called E\(^+\)-logics; those that do not are called E-logics. The following specification rule is valid in E\(^+\)-logics but not in E-logics: