Phase, in sound theory at least, describes the time relationship between two or more waveforms. Phase is measured in degrees, and can be easily demonstrated using sine waves. A sine wave starts when its cycle starts. This is denoted by 0◦. One complete cycle signiﬁes 360◦ (which is also 0◦), half a cycle 180◦, quarter of a cycle 90◦ and so forth. If one sine wave reached quarter of its cycle when a cycle begins on an identical waveform, the two are said to be 90◦ out of phase (Figure 11.1).