A simple electromagnetic wave, graphing its fluctuating strength against time, has the shape of an ordinary sine wave. With sine waves it is customary to speak of a completed cycle as advancing through 360 degrees, going full circle. Starting from zero, then, the first crest occurs a quarter of the way through the cycle, at 90 degrees; the wave passes through zero again, at 180 degrees, before cresting in the opposite direction, at 270 degrees. Reaching zero for the second time, at 360 degrees, the cycle repeats. Expressed in this way, in degrees, any point in a wave's cycle is called a *phase angle*.

When several waves meet, they add together. If two waves of the same height happen to be in phase, reaching the same crests at the same time, the result is a wave of identical frequency but of twice the amplitude. Should they crest in opposite directions at the same time—180 degrees out of phase—they cancel exactly and disappear. In real-world cases, waves of many frequencies, amplitudes, and relative phases are always being thrown together, resulting in complex waveforms. As a practical and mathematical matter, however, it is always possible to separate an irregular, jumbled form into its pure sine-wave components.