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A study has been made of the effect of crystal-blank and electrode geometries on the performance of AT-cut and BT-cut quartz-crystal units intended for use in filters. Methods for controlling the frequencies and activities of unwanted modes of vibration are described, and it is shown that, by scaling the geometries, it is possible to translate crystal designs from one frequency to another. At frequencies below about 10 Mc/s, the unwanted spectrum of inharmonic thickness-shear resonances can be modified by the application of acoustically lossy material and the vacuum deposition of metal layers on to regions of the crystal which are most active for these modes of vibration. Above 10 Mc/s, it is found that the motional area of the crystal does not extend far outside the central electroded area, and, in view of this, it is possible to have more than one resonant section on the miniaturisation. Frequency/temperature between thickness-shear vibrations and influenced by the electrode thickness and same quartz blank, thereby obtaining a considerable degree of and activity/temperature characteristics, illustrating coupling high overtones of low-frequency resonances, are shown to be the method of mounting the crystal.