From the trig identity , we have
From this we may conclude that every sinusoid can be expressed as the sum of a sine function (phase zero) and a cosine function (phase ). If the sine part is called the “in-phase” component, the cosine part can be called the “phase-quadrature” component. In general, “phase quadrature” means “90 degrees out of phase,” i.e., a relative phase shift of .
It is also the case that every sum of an in-phase and quadrature component can be expressed as a single sinusoid at some amplitude and phase. The proof is obtained by working the previous derivation backwards.