One cycle of a waveform is considered to have 360 degrees, just like a circle. How far you move around the circle (or through the waveform) can be defined by the *phase*. For example, if you are one-quarter of the way through a waveform’s cycle, your phase is 90°.

Where this becomes interesting is that there are some circuits that can alter the phase of a signal, advancing or retarding it by a certain number of degrees. If you advance a waveform 180° or a half cycle, in many cases – particularly a sine wave, which is the purest component of a sound – it looks like an inverted copy of the original waveform. Add a waveform to an inverted copy of itself, and they cancel each other out, resulting in silence. As a result, *phase cancellation* is a very important phenomenon.