Who gets mark-room at a mark?

Rule 18 applies when boats are required to leave a mark on the same side and at least one of them is in the zone (the area within three hull lengths of the mark). It does not apply between boats on opposite tacks on a beat to windward, between a boat approaching a mark and one leaving it, or when the mark is a continuing obstruction (rule 19 applies then).

When the first of two boats reaches the zone:

• If the boats are overlapped, the outside boat at that moment must give the inside boat mark-room.
• If they are not overlapped, the boat that has not yet reached the zone must give the other boat mark-room.

The boat required to give mark-room keeps doing so for as long as the rule applies, even if the overlap is later broken or a new one begins. If there is reasonable doubt that a boat obtained or broke an overlap in time, it is presumed she did not.

Source: Racing Rules of Sailing 2025-2028 with Australian Sailing prescriptions, rule 18 and the definition of Zone.