The quartz clock has made possible a notable improvement in the precision of timekeeping, with the additional advantage of providing a standard of frequency as well. This is perhaps putting the cart before the horse, for the quartz crystal oscillator was developed in the first instance as a precision standard of frequency; it was then adapted, by frequency subdivision and the use of a low frequency output for driving a phonic motor from which seconds impulses could be taken, to serve as a clock. It is primarily from its use as a standard of frequency that it is of navigational interest, for various radio aids to navigation, such as Gee, Loran, Consol and Decca depend upon accurately standardized frequencies. For the satisfactory checking of a frequency, comparison must be made with a standard whose accuracy is at least one degree higher than that of the frequency which has to be checked. Calibration laboratories, in turn, require to check their own standards. As the rotation of the Earth provides our fundamental unit of time, it follows that time, and therefore also frequency, must be determined from astronomical observations; hence it is necessarily the responsibility of the Royal Greenwich Observatory to provide both time and frequency with an accuracy sufficient for all practical requirements. The practical requirements have become more stringent in recent years, and that is why great efforts have been made to improve the precision of time determination and of timekeeping at the Observatory.