The Regulations for Preventing Collisions at Sea were formulated before the invention of radar, and radar has invalidated one of the principles upon which the Rules are based. This principle is that when two vessels are in risk of collision one shall get out of the way, and the other shall stand-on (except in the cases where two vessels are meeting end-on, and where the stand-on vessel finds herself so close that collision cannot be avoided by action of the giving-way vessel alone). This assumes that if one vessel can see another she also can be seen, and this is of course likely to be the case where visual sighting is concerned. From radar observations one cannot know if the other ship has observed you, and it is unlikely that we shall know this with certainty, for radar and other aids may fail. Providing there is sufficient sea-room and not too much traffic, this drawback can usually be overcome by taking early and substantial avoiding action (as recommended in the Annex to the Rules). If one ship makes a manœuvre which cancels the avoiding action taken by the other there is still time for second thoughts; but in crowded waters there may not be so. Even when both vessels are navigated correctly, the imprecision of radar predictions may lead to dangerously close passings.