When using radar in thick weather as an anti-collision warning device it is necessary, when planning any manœuvre, to consider the effect of the proposed alteration of own vessel's course and/or speed on the PPI presentation of the target vessel's radar, should she be using it. An alteration of course or speed of own vessel will alter the direction or speed of the apparent motion of the spot on the target vessel's PPI. As the apparent motion on relative plots kept by the two vessels are reciprocal images of each other it is a simple matter to determine, in any particular case, the effect on the target's plot of own vessel's manœuvre by considering the effect on the apparent motion on own vessel's plot.