(a) Rule 18 applies between boats when they are required to leave a mark on the same side and at least one of them is in the zone.  However, it does not apply 
   (1) between boats on opposite tacks on a beat to windward,
   (2) between boats on opposite tacks when the proper course at the mark for one but not both of them is to tack.
   (3) between a boat approaching a mark and one leaving it, or
   (4) if the mark is a continuing obstruction, in which case rule 19 applies.

(b) Rule 18 no longer applies between boats when mark-room has been given.

(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 other boat 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) Rules 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 a reasonable doubt that a boat obtained or broke an overlap in time, it shall be presumed that she did not. 

If a boat passes head to wind from port to starboard tack  in the zone of a mark to be left to port, rule 18.2 does not apply between her and another boat on starboard tack that is fetching the mark.  If the other boat has been on starboard tack since entering the zone, the boat that passed head to wind
(a) shall not cause the other boat to sail above close-hauled to avoid contact, and
(b) shall give mark-room if the other boat becomes overlapped inside her. 

When an inside overlapped right-of-way boat must gybe at a mark to sail her proper course, until she gybes she shall sail not further from the mark than needed to sail that course.  Rule 18.4 does not apply at a gate mark.
 

Rule 19 applies between two boats at an obstruction except when rule 18 applies between them and
(a) the obstruction is a mark, or
(b) the obstruction is another boat overlapped with each of them.

However, at a continuing obstruction, rule 19 always applies and rule 18 does not.

(a) A right-of-way boat may choose to pass an obstruction on her port or starboard side.  If a right-of-way boat changes course when choosing on which side to pass the obstruction, she shall give the other boat room to keep clear.
(b) When boats are overlapped, the outside boat shall give the inside boat room between her and the obstruction, unless she has been unable to do so from the time the overlap began.
(c) While boats are passing a continuing obstruction, if a boat that was clear astern and required to keep clear becomes overlapped between the other boat and the obstruction and, at the moment the overlap begins, there is not room for her to pass between them, 
   (1) she is not entitled to room under rule 19.2(b), and
   (2) while the boats remain overlapped, she shall keep clear and rules 10 and 11 do not apply.

A boat may hail for room to tack and avoid a boat on the same tack by hailing 'Room to tack'. However, she shall not hail unless

In addition, she shall not hail if the obstruction is a mark and a boat that is fetching it would be required to change course as a result of the hail.

A boat sailing towards the pre-start side of the starting line or one of its extensions after her starting signal to start or to comply with rule 30.1 shall keep clear of a boat not doing so until her hull is completely on the pre-start side.

 A boat taking a penalty shall keep clear of one that is not.

A boat moving astern, or sideways to windward, through the water by backing a sail shall keep clear of one that is not.

 If possible, a boat shall avoid a boat that is capsized or has not regained control after capsizing, is anchored or aground, or is trying to help a person or vessel in danger.  A boat is capsized when her masthead is in the water.

 If reasonably possible, a boat not racing shall not interfere with a boat that is racing.
  

If reasonably possible, a boat shall not interfere with a boat that is taking a penalty, sailing on another leg or subject to rule 22.1. However, after the starting signal this rule does not apply when the boat is sailing her proper course.