Abstract: An AT-cut piezoelectric crystal resonator having a plurality of contoured surfaces, two of these contoured surfaces being generated by translating contour lines parallel to the z-axis and perpendicular to the electric axis. At least one of these two surfaces is convex, and another two contoured surfaces are perpendicular to the z-axis and spaced apart by a length dimension z'.sub.0 which is 0.1 to 1.3 of the maximum thickness dimension representing the distance between the first mentioned two surfaces and measured along a line perpendicular to the x-axis. The resonator is adapted for performing a mode of vibration along the thickness forming either a fundamental standing wave or an overtone thereof of a thickness-shear vibration. The free ends of the vibration are positioned at the first mentioned two surfaces, and the amplitude of vibratory displacements appearing along the x-axis is maximum at the center of each surface of the first mentioned two surfaces.