The simplest configuration considered in Ref. 1 consists of an interface between an isotropic material with permittivity ε and a uniaxial crystal with permittivities ε 0 and ε e for the ordinary and the extraordinary waves respectively. The crystal C axis is parallel to the interface. For this configuration, the DSW can propagate along the interface within certain angular intervals with respect to the C axis, provided that the condition of ε e > ε > ε 0 is satisfied. Thus DSW are supported by interfaces with positive birefringent crystals only (ε e > ε 0). The angular interval is defined by the parameter