Abstract : By a theorem of E.Kunz, an irreducible reduced curve is Gorenstein if and only if its semigroup satisfies a certain symmetry property. This characterization of Gorenstein curves is extended to curves with several components by F. Delgado. The purpose of this talk is to show that an analogous symmetry is satisfied between the sets of values of a fractional ideal and the values of its dual. This property has been later extended by P. Korell, M. Schulze and L.Tozzo to more general one dimensional rings called admissible rings, which include in particular all rings of reduced reducible curves. We will also give several consequences of this symmetry concerning a special kind of elements called maximals, and concerning the coefficients of Poincaré series with respect to the multi-index filtration.