When manœuvring ships, mariners usually pay a great deal of attention to the rate at which the heading changes but the direction in which the ship actually moves may differ from the direction in which she is heading. This difference may be caused not only by current but also by the ship's drift velocity through the water in a direction perpendicular to the heading. This velocity is hard to recognize but can cause a turning moment on the ship, whether this is desired or not. In difficult manœuvres which cannot always be accomplished by changes of heading only, one can use the effects of this drift velocity. The drift angle, the angle between the heading and the direction in which the centre of gravity moves through the water, must not be confused with the course allowance necessary to make good a track over the ground when there is a current.