**2.3.1.2 B-Frame**

The b-frame, also known as the body frame, is a moving reference frame that has its origin fixed to the body of a vehicle. Due to various properties that exist at different points within the body of the vehicle, it is convenient to place the origin of this frame at one of these points to take advantage of, for example, body symmetries, centre of gravity, or centre of buoyancy. As a general rule, the *x*-axis of this frame points from aft to fore along the longitudinal axis of the body, the *y*-axis points from port to starboard, and the *z*-axis points from top to bottom. Due to the orientation of this frame, it is appropriate to express the velocities of the vehicle in this frame.
