Who gets to go inside at a mark?

• The zone: the area around a mark within three hull lengths of the boat nearer to it. A boat is in the zone when any part of its hull is.
• When the rule applies: between boats required to leave a mark on the same side, when at least one of them is in the zone.
• The snapshot: when the first of two boats reaches the zone, if they are overlapped the outside boat at that moment gives the inside boat mark-room. If they are not, the boat that has not reached the zone gives it.
• It lasts: the duty continues even if the overlap is later broken or a new one begins. It ends if the boat entitled to mark-room passes head to wind or leaves the zone.
• What mark-room is: room to sail to the mark when that is the boat's proper course, to round or pass it on the required side and to leave it astern.
• Doubt: if there is reasonable doubt that a boat got or broke an overlap in time, it is presumed that it did not.
• A late inside overlap: if a boat got an inside overlap from clear astern and the outside boat has been unable to give mark-room from the time the overlap began, the outside boat does not have to.

When the rule does not apply:

• between boats on opposite tacks on a beat to windward;
• between a boat approaching a mark and one leaving it;
• at a starting mark surrounded by navigable water, when boats are approaching it to start.

Tacking in the zone: a boat that tacks from port to starboard inside the zone of a mark to be left to port gets no mark-room from a starboard-tack boat that is fetching the mark, and must not make that boat sail above close-hauled to avoid contact.

Every boat wants the same few metres of water at a mark, and they arrive at different angles. Fixing the rights at one moment, entry to the zone, means crews know where they stand before the turn begins and gives nobody a reason to force a way in at the last second.