CRYSTAL VIBRATORS and atomic frequency standards are continuing to attract the interest of engineers and physicists. In particular, the energy-trapping concept has assisted in the application of theory to practical quartz resonators. Thus filter crystals have been extended to 125 MHz. Similarly, multiple-mode resonators of piezoelectric ceramic and quartz can now be made on a single crystal wafer, making possible a filter in a monolithic integrated form. Crystal oscillators, too, have been made, in which the crystal serves both as a control element and as an active device.