Consider the equation (1-v2)uxx+2(1+v2)uxy+(1-v2)uyy = 0 where v is a constant real parameter with |v| < 1. After proving that this is an hyperbolic PDE show that its general solution (without Cauchy data) is similar to the general solution of the wave equation (uxx - v2uyy) with v equal to the propagation speed.