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A new approach to the design of crystal units for primary frequency standard use has resulted in crystal units in the 3- to 20-mc frequency range characterized by high Q and low capacitance in the series arm of the equivalent electrical circuit. By utilizing the overtone frequency of specially shaped AT-cut quartz plates, both Q and the rate of impedance change with frequency are enhanced together, and in addition the stability with time of the crystal unit is increased because of a larger frequency-determining dimension. Additional characteristics of the crystal units include small size, stability under conditions of vibration and shock, and low-temperature coefficient. Crystal-oscillator stabilities of one part in 108 per month have been achieved without recourse to stabilized circuits.