A rib-like waveguide structure consisting of three dielectric films is analyzed, in which properly chosen film thicknesses allow the lateral containment of any selected waveguide mode, with the unselected modes radiating laterally away from the rib structure. The wave-guide consists of two high-index films separated by a low index layer in which the fields are evanescent. The lateral guidance method mechanism arises from the joint dependence of the planar waveguide mode propagation constants on the strength of evanescent coupling between the two high-index films, and on the propagation constant difference between the planar wave-guide mode and one with exactly resonant coupling between the high-index layers. Depending on the mode and the film thicknesses, the radiation is guided along a ridge or a trough in the thickness of the low-index layer. Unlike mode-stripping structures based on preferential absorption of the unwanted modes, this all-dielectric structure introduces no loss to the selected mode, other than the unavoidable scattering and residual absorption in the dielectric media, and the reflection and radiation loss at one required longitudinal inteface between adjacent waveguide sections. Also, unlike mode-selecting structures based on Bragg effects, no maintenance of phase-matched conditions over controlled propagation lengths is required.