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Results from detailed numerical analyses of the modal characteristics of large-cross-section silicon-on-insulator-based rib waveguides are presented. They highlight for the first time that satisfying widely used design criteria is not sufficient to ensure single-mode behavior. In particular, the geometries that the design formulas predict should be single-mode are shown to support higher order vertical modes that do not couple (leak) into the outer slab region and are thus low loss in nature. Fortunately, a wide range of practical rib geometries still remains for which the leakage loss of modes other than those of EH00 and HE00 is sufficiently high to make the waveguides effectively single mode for each polarization.