The cut angle of the crystal determines the vibration mode. “Flexural” mode with X cut has side-to-side oscillations enabling this tuning-fork shaped crystal unit to cover frequencies in the kHz band, such as 32.768kHz, which is commonly used as a clock frequency. “Thickness shear” mode with AT-cut has the top and bottom surfaces of the crystal chip move in opposite directions enabling it to cover frequencies ranging up to several hundred MHz. The stability vs. temperature curve of both modes can be characterized by the respective equations above.