A solution for coupling through a double slit between two parallel-plate waveguides is obtained in analytic series form. The Fourier transform is used to express the scattered field in the spectral domain in terms of waveguide modes. The simultaneous equations for the modal coefficients are solved to obtain the transmission and reflection coefficients in simple, rigorous, and numerically efficient series. Numerical computations are performed to illustrate the coupling and directivity characteristics of a double-split coupler. (C) 2000 John Wiley & Sons, Inc.