(a) When the first of two boats reaches the zone,
(1) if the boats are overlapped, the
outside boat at that moment shall give the inside boat mark-room;
(2) if the boats are not overlapped,
the boat that has not reached the zone
at that moment shall give the mark-room.
When a boat is required to give mark-room
by this rule, she shall continue to do so for as long as this rule applies,
even if later an overlap is broken
or a new overlap begins.
(b) Rule 18.2(a) no longer applies if the boat entitled to mark-room passes head to wind or leaves the zone.
(c) When rule 18.2(a) does not apply and the boats are overlapped, the outside boat shall give the inside boat mark-room.
(d) 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, rules 18.2(a) and 18.2(c) do not apply between them.
(e) If there is reasonable doubt that a boat obtained or broke an overlap in time, it shall be presumed that she did not.