So, you see, if I want to go from here to here trace it, first of all I must define a fix coordinate system at some point on the body, which can be anywhere and how much thus displacements are depends on where the coordinate is, see that is very important therefore, if I say the ship has roll so much, it is really not complete unless you said about which axis, so this is the point I am trying to say. So, the definition of ship motion therefore, basically since my objective is to find out here to here, all I need to first define