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The diffraction of a plane electromagnetic wave by two identical slits in tandem is investigated for normal incidence with the polarization parallel to the edges of the slits. Theory shows that the scattering cross-section coefficient is proportional to the imaginary part of the far field forward scattering factor. The stationary form of the scattering cross section is developed in terms of the incident field and unknown currents on the edges of the conductors forming the slits. Calculations using the Kirchoff-type approximation in this stationary form for a tandem slit separation of are in good agreement with the experimental values for slit widths greater than . Similar calculations for zero tandem slit separation, corresponding to a single slit, and for slit widths greater than are in good agreement with those of the exact theory of Morse and Ruberstein, and as determined experimentally. The infinitely long slits are approximated experimentally by use of a parallel plate system described earlier by Row. The experimental results show an interesting resonance effect as the slit width changes.