This paper discusses the concept of using two single-mode resonators to construct a quasi-dual-mode resonator that has all the features of traditional dual-mode resonators. Two novel filter structures are presented in this paper that employ this concept; one employs planar-type resonators and the other employs dielectric resonators. A new configuration for a single-mode dielectric resonator is also proposed in this paper, The use of such a type of resonator makes it possible to construct dielectric-resonator filters that are much smaller in size than conventional TE/sub 01/ single-mode resonators. Measured results are presented for four filters along with theoretical results to verify the novel concepts proposed in this paper.