For the description of rigid body motions, it is more convenient to use the center of gravity of the body, or the combined COG of mass of the structure and the liquid in the internal tanks, as a dynamic reference point. The local structure axes (LSA), body fixed axes, or Gxyz, is defined for each individual structure. As shown in Figure 1.1: Definition of Axis Systems, the origin of the local structure axes is at at this dynamic reference point. The local structure axes through the origin will initially be parallel to the fixed reference axes.