Atomic and molecular vibrations constitute the most regular of motions in nature, but most are at such a high frequency as to be unusable as time standards. Radio-frequency techniques are used to generate energy at the frequency of the atomic transition from a lower frequency generated by a quartz-crystal oscillator, while the atomic vibration controls the absolute frequency of oscillation, frequency division by electronic circuits being used to obtain the low frequency required to drive the clocks indicating absolute time. Accuracy to less than one second error in 100,000 years is now available with the caesium atomic clock.