From my initial reading of Rule 18 and study of case studies, I was always under the impression that Rule 18 only applied to boats on the same tack, but it now seems I was wrong. I especially thought that if boats were approaching a downwind mark, and were on opposite tacks, that the boat on starboard had right of way and didn't need to give room to the boat on port, even if it was inside and overlapped. But an example a the excellent site http://uksailmakers.com/RulesQuiz/, Quiz 2 suggests that a starboard boat has ROW because it is inside and overlapped, not just because it is on starboard. In that example, if the boats had to round the mark to starboard instead (we have a mark just like this on our river course), I guess the port tack boats would claim room over the starboard boat ? Reading 18.1b again now seems to make this pretty clear, Rule 18 applies to boats on opposite tacks when they are not on a beat to windward. Its a common suggestion to sail downwind on starboard to get ROW but I guess this stops applying when in the 2 boat length circle ? Have I got it right now ?