At one time this term would have been applied to one kind of clock only, the synchronous electric clock. The synchronous electric clock is not strictly a true clock; it has an electric motor synchronised to the frequency of the public power supply with a reduction train to give a time indication. Today there are electric clocks driven from the main power supply which count the number of electrical oscillations and display a time indication based on the total elapsed. Again, these are not clocks as there is no self-contained time measuring device, and they are connected to the mains principally to use the frequency as a time source. The greatest disadvantage of the mains clock is that it must be connected to the mains supply by an electric lead from a suitable power point. Alternating current supplies with strict frequency control are necessary for accurate time indication.