Rule 18.1
When Rule 18 Applies
(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.