A longitudinally-coupled resonator (LCR) filter, also called a DMS filter, consists of either two or three interdigital transducers located between two strongly-reflecting gratings. These devices have been widely used as low-loss filters at RF frequencies. The behaviour of the devices is complicated by the fact that the transducers have strong internal reflections. This paper considers the two-transducer type and shows that, with some realistic assumptions, the device behaves much like a simple resonator. In the passband, the filter response can be expressed in a simplified manner which gives much insight into the physical operation of the device. With appropriate electrical matching, the resonant behaviour disappears and the device behaves as a delay line. The matching required for a flat low-loss response is deduced. Equivalent circuits for two- and three-transducer devices are derived.