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Various publications address the suppression of the undesired mode coupling in laterally bounded conductor-backed coplanar waveguides (CPWs). In this paper, we present an integrated waveguide structure in which this coupling effect of different guided modes is deliberately exploited. Efficient forward coupling at arbitrary levels is achieved between a substrate integrated rectangular waveguide (SIW) and a CPW on its broad wall. Such hybrid integrated couplers combine the SIW's low-loss property with the CPW's flexibility and monolithic microwave integrated circuit compatibility. They are attractive for transitions, six-ports, antenna feeding networks, active antennas, and distributed phase shifters or amplifiers in millimeter-wave applications. The coupling mechanism is studied and a rapid design becomes feasible by use of the provided theory. Two couplers (a 3- and 0-dB version with 4.95- and 9.8-mm-long coupling sections) were designed and fabricated on alumina substrate for operation around 50 GHz. The 3-dB coupler achieves a wide relative bandwidth of 25% at 0.5-dB amplitude imbalance due to the asymmetric coupling structure, whereas the 0-dB coupler (transition) has a 7% relative bandwidth at less than -0.2-dB coupling loss. Results obtained from millimeter-wave probe measurements on realized prototypes agree well with our theory and simulations.