Question
X and A are approaching a starboard-hand windward mark, overlapped and level, on port tack. A, to leeward, is on the layline. X is on collision course with her team mate Y who is on starboard tack. X hails A for room to pass astern of Y. A gives her room, but as a result is forced to pass the wrong side of the mark.
A protests. What should the call be?

Answer
With respect to the windward mark, A is inside boat and has right of way over X, so X must keep clear of A and give her room to round the mark. With respect to Y, A and X both have to keep clear of Y, so Y ranks as an obstruction to both. At position 2 A and X are both about to pass the same side of Y, so rule 18.2(a) applies. A is outside right-of-way boat and must give X room to pass Y.
Once A has given X room to pass Y, X's obligation is to continue to keep clear of A. If A is able to pass the mark, rule 18 applies between A and X, and A may luff as she pleases to round the mark on the correct side. However, because A is unable to pass the mark on the correct side, rule 18 no longer applies. X’s only obligation is to keep clear, which she does. No penalty.
Rule 19.1, Room to Tack at an Obstruction, does not apply when A and X meet Y, because A does not herself have to make a substantial change of course to avoid Y.