A state vector may be expressed in a variety of coordinate systems. Selection of a coordinate system depends on what makes sense for the problem being solved. In aviation, a state vector may be expressed in the North, East, Down coordinate system, where North position is expressed as a latitude, East position as a longitude, and Down represents the inverse of altitude. (Down is used instead of Up to conform to the right-hand rule. The velocity components of the state vector would be North velocity, East velocity, and vertical speed.