As the diagram shows, Stan had been on starboard for several lengths before he reached the zone. He was sailing close-hauled and fetching the mark. Ian and Marge entered the zone on port. Marge could’ve hailed Ian for “Room to tack” under Rule 20, but she did not. After Position 2, Ian decided he could tack and fetch the mark. Ian passed head to wind and Marge then did the same. As Ian and Marge tacked, Stan continued to sail closehauled, and there was space for one boat, not two, to pass between Stan and the mark. Between Positions 4 and 5, Ian luffed to round the mark. Marge luffed in response and caused Stan to sail above closehauled. There was no contact, and no boat took a Two-Turns Penalty. Stan and Marge both protested Ian.
The case answers two questions: How does Rule 18.3 apply to this incident, and what should the decision be? After Ian and Marge turned past head to wind, each of them was on the same tack as Stan who was fetching the mark. Therefore, Rule 18.3 applied between Ian and Stan and between Marge and Stan. When Ian and Marge luffed between Positions 4 and 5, they caused Stan to sail above closehauled to avoid contact, and therefore both broke Rule 18.3(a). Marge, however, was compelled to luff by Ian’s luff, and so, under Rule 64.1(a), Marge was exonerated for breaking Rule 18.3(a).