The phase difference is especially important when comparing a periodic signal F with a shifted and possibly scaled version G of it. That is, suppose that G ( t ) = α α F ( t + τ τ ) for some constants α α , τ τ and all t . Suppose also that the origin for computing the phase of G has been shifted too. In that case, the phase difference φ φ is a constant (independent of t ), called the 'phase shift' or 'phase offset' of G relative to F . In the clock analogy, this situation corresponds to the two hands turning at the same speed, so that the angle between them is constant.