Every digit set formula_1 has a dual digit set formula_44 given by the inverse order of the digits with an isomorphism formula_45 defined by formula_46. As a result, for any signed-digit representations formula_47 of a number system ring formula_48 constructed from formula_1 with valuation formula_50, there exists a dual signed-digit representations of formula_48, formula_52, constructed from formula_44 with valuation formula_54, and an isomorphism formula_55 defined by formula_56, where formula_57 is the additive inverse operator of formula_48. The digit set for balanced form representations is self-dual.