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A class of frequency-selective networks is described that combines the properties of a directional coupler and a conventional filter. These networks, which will be called "directional filters," are particularly suited for combining or separating signals of different frequencies in communication systems, for multiplexing several equipments connected to a single antenna, etc. A signal entering one arm of the network will be transferred with essentially zero loss to a second arm over a desired channel of frequencies, while at other frequencies the signal will emerge with little loss from a third arm. Over the entire frequency range the fourth arm is isolated and the first arm is nonreflecting. Many new types of directional filters have been found and are described in this paper. Several of these are particularly easy to design and build in strip or coaxial line, while others lead to compact, economical structures in waveguide. Several lumped-constant circuits are also given. Design information and experimental data are included in this paper.