Abstract: An energy-trapping thickness-shear resonator includes a piezoelectric substrate, a first electrode extending from a first end to a central portion of a first surface of the piezoelectric substrate and a second electrode extending from a second end to the central portion of a second surface of the substrate. Thus, the resonator utilizes thickness shear vibrations generated in a portion of the piezoelectric substrate located between the first and second electrodes at the central portion of the substrate. The width W of the la substrate is set to be equal to or less than about 0.7 mm and also to satisfy both of the following expressions (1) and (3) or to satisfy both of the expressions (2) and (3):(1) 0<W<k T/(1-a)(2) k(2n-1)T/(1-a)<W<k(2n+1)T/(1-a)(3) 0 21 .alpha.<(fa-fr)/fawhere T indicates the thickness of the piezoelectric substrate; k is 1/2; f.sub.OSC equals fa (1-.alpha.); f.sub.