The coupling of two parallel transmission lines is utilized in directional couplers and filters which are simple to construct and have excellent electrical characteristics from high frequencies to microwave frequencies. The paper establishes the mathematical equivalence of such couplers to transmission-line stepped-impedance filters. It also places new demands on that theory in so far as new insertion-loss functions, having, for instance, equi-ripple characteristics in the stop-band, will be required. A new directional filter circuit, or channel-dropping filter, is suggested.