when you say a wave which is 5 meter high, actually the point is not always at 5 meter, no when this point is a 5 meter, this is actually some other height. See, if you look at a point, it goes from minus 5 meter up to 5 meter and then close down and goes up etcetera, that is what it does. The form moves up and down from minus 5 meter to 5 meter. Xi is actually for any point free surface height. So, xi becomes maximum as equal to xi a and height is twice the xi a because it is minus xi. This I will come later on, this is the kind of an example, try to have to prove that. So, it turns out that the fact that we use, that the wave height is sinusoidal, the wave is sinusoidal form of it is not really without foundation.