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The distributed capacitance of the periodic grating, the 2- electrode, and the 3-electrode stripline waveguide modulators are derived exactly using conformal mapping techniques. The capacitance formulas are shown to have a common form; they differ in a single parameter which is simply related to the electrode width and spacing. Discrepancies previously found in the capacitance expressions used for the 2-electrode structure are resolved. For the finite 3-electrode structure studied here, a self- and a mutual distributed capacitance are defined. Techniques for switching a 3-electrode directional coupler are suggested. The closed form mapping function derived here will be useful in the design of waveguide directional couplers. The electrode structures examined here are similar to those used in surface acoustic wave devices; therefore, the capacitance calculations should also be applicable.