The subject of frequency control is closely bound up with that of timekeeping where the primary requirement is for a device or system to play the part of a pendulum. At the lower end of the frequency spectrum in the audio range up to several hundred megacycles, the “pendulum” consists most often of a mechanical resonance structure such as a tuning fork or quartz crystal vibrator whose frequency is determined from the classical theory of elasticity. The other major division of the frequency-control field is occupied by devices that utilize quantum-mechanical principles for their operation, where the “pendulum” is identified with an atomic or molecular resonance, and where the frequency, consequently, is in the gigacycle range.