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Equations and curves giving cutoff frequency and impedance are presented for rectangular wave guide having a rectangular ridge projecting inward from one or both sides. It is shown that ridge wave guide has a lower cutoff frequency and impedance and greater higher-mode separation than a plain rectangular wave guide of the same width and height. The cutoff frequency equation is fairly accurate for any practical cross section. The impedance equation is strictly accurate only for an extremely thin cross section. Values found by the use of this equation have, however, been found to check experimental values very closely. A number of uses for this type of wave guide are suggested.