# Translations:Phase velocity/22/en

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The term on the left represents the velocity of propagation, subject to the condition of constant phase. Choose any point on the wave profile - for example, the crest of the wave. As the wave moves through space, the displacement *u* of the crest remains constant. Because the only variable in the sinusoidal wave function is the phase, it too must be constant. That is, the phase is fixed at a value producing the constant displacement *u* at the chosen point. The point moves along with the profile at velocity *v*, and so does the constant phase condition.