# Translations:Phase velocity/26/en

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Let us consider further the concept of propagation at constant phase and examine how this concept governs the propagation of any one of the sinusoidal waves, say, ). Assume that the quantity *v* in this equation is positive, and choose the negative sign. Then the condition of constant phase is that . This equation says that as *t* increases, *x* also must increase. Even when , so that , the quantity *x* must increase - that is, it will become less negative. Thus, the constant-phase condition implies propagation in the increasing *x*-direction. Let us once more assume that *v* is positive, but now we choose the negative sign in the constant-phase condition. We get