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High-precision quartz crystal resonators were recently added as a part of the primary standard of frequency at the National Bureau of Standards. Their reliability over short and long periods of time resulted in establishment of a frequency and time reference system constant to 1 part in 1010 per day. Measurement equipment and methods, are described in detail. The equivalent circuit for the crystal unit is discussed and the effects of external influences such as stray capacitance, electric and magnetic fields, connecting cables, ambient temperatures, and amplitude of vibration are considered. An assessment method is discussed. This method is based on the fitting of a natural aging or drift curve for each resonator to an observed curve after sufficient performance data are obtained. The use of resonators with crystal clocks represents one of the simplest and most reliable and economical methods of establishing a precise frequency reference in terms of mean solar time and of noting deviations in the earth's rate of rotation.