18.2 Giving Mark-Room
(a) When boats are overlapped the outside boat shall give the inside boat mark-room, unless rule 18.2(b) applies.
(b) If boats are overlapped when the first of them reaches the zone, the outside boat at that moment shall thereafter give the inside boat mark-room. If a boat is clear ahead when she reaches the zone, the boat clear astern at that moment shall thereafter give her mark-room.
(c) When a boat is required to give mark-room
by rule 18.2(b),
(1) she shall continue to do so even if later an overlap is broken or a new overlap begins;
(2) if she becomes overlapped inside the boat entitled to mark-room, she shall also give that boat room to sail her proper course while they remain overlapped.
However, if the boat entitled to mark-room passes head to wind or leaves the zone, rule 18.2(b) ceases to apply.
(d) If there is reasonable doubt that a boat obtained or broke an overlap in time, it shall be presumed that she did not.
(e) If a boat obtained an inside overlap from clear astern or by tacking to windward of the other boat and, from the time the overlap began, the outside boat has been unable to give mark-room, she is not required to give it.