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A general discussion of a wire-coupled crystal mechanical filter and its problem on the design and fabrication is followed by a detailed description of transducer-coupler-transducer system named the two-element type, which is the simplest construction of electromechanical filters. This filter is composed of quartz crystals as the transducers and resonators and fine wires as the couplers. The theoretical possibilities of this construction are discussed by the longitudinal-vibration type and its nature is examined by the several experiments. Then, a filtering characteristic of a flexural-vibration type filter is illustrated as an example of filters constructed by the other vibration modes. The limits of usuable frequency ranges are approximately from 10 to 1000 kc in view of their fabrication easiness and the preferred fractional bandwidths are from 0.05 to 3 per cent.