Oscillation, in general, is a periodic fluctuation between two things; in the broadest sense, oscillation can occur in anything from a person's decision-making process to tides and the pendulum of a clock. Oscillation in a device called an oscillator is usually a back and forth motion over a central neutral point, created by changes in energy. In a pendulum-driven clock, for example, the oscillation is the back and forth movement of the pendulum. Oscillators may be mechanical or electronic, but all work on the same principles. Other devices based on the principles of oscillation include the oscillograph and the oscilloscope. Like other oscillators, a clock pendulum's oscillation is maintained by changes in energy. In this case, potential energy, present when the pendulum is at the top of its swing, is converted to kinetic energy as the pendulum falls and is driven upwards on the other side. When the kinetic energy has been spent, at the top of the swing, the pendulum's energy is potential once more. With no kinetic energy to drive it higher, the pendulum falls. A pendulum clock keeps time according to the frequency of the pendulum's swing (the number of times it swings per second). Friction would eventually cause the movement to stop, but mechanical pendulum clocks use a spring to help the device overcome friction's drag. Most modern timepieces use quartz or electronic oscillators. The most accurate timepiece in the world, the atomic clock, measures time according to the oscillation within atoms