An analysis of heterophase states was carried out for ferro-active solid solutions of the system (1?x?y)NaNbO3?xKNbO3?yCdNb₂O₆. A complete stress relief at interfaces between phases in solid solutions with y=0.05 and x=0.10, 0.40, and 0.45 is realized at the single-domain state of the tetragonal and orthorhombic phases. In solid solutions with y=0.05 and x=0.05 and 0.20, as well as with y=0.075 and 0.10, and x=0.15, the formation of a planar interphase boundary being parallel to a zero-net-strain plane occurs at the elastic matching of the single-domain tetragonal and polydomain orthorhombic phases. In contrast to this, at y=0.05 and x=0.25, 0.30, and 0.35, the complete stress relief is associated with a co-existence of the single-domain orthorhombic and polydomain tetragonal phases, however in some cases, the tetragonal phase aims at a monodomainization.