Of a periodic wave, the number of suitable units of angular measure between a point on the wave and a reference point. Note 1: The reference point may be a point on another periodic wave. The waves may be plotted on a suitable coordinate system, such as a Cartesian plot, with degrees or other angular measure usually plotted on the abscissa and amplitude on the ordinate. Usually, at least one full cycle of each wave is plotted, with 360&176; (2 radians) encompassing one full cycle. The reference points may be any significant instants on the waves, such as where they cross the abscissa axis. Note 2: The use of angular measure to define the relationship between a periodic wave and a reference point is derived from the projection of a rotating vector onto the real axis of an Argand diagram. Note 3: The value of the phase angle of a point on the wave is the point on the abscissa that corresponds to the point on the wave. Note 4: The phase angle of a vector may be written as M, where M is the magnitude of the vector and is the phase angle relative to the specified reference.