Weather-mark pile-up
The situation
A pack of Optis converges on the windward mark on starboard. As the leaders reach the three-length zone, Kai is overlapped on the inside of the boat next to him; behind, boats are stacked up.
The question
What is Kai entitled to, and when is it decided?
The ruling
If Kai is overlapped on the inside when the first of them reaches the zone, the outside boat must give him mark-room — room to sail to the mark and round it (rule 18.2(a)). It's locked in at the moment the first boat reaches the zone and isn't lost if the overlap later breaks. But everyone is still bound by the other rules: Kai can't take more room than he needs, and on opposite tacks on the beat rule 18 switches off, so a port-tack boat there gets nothing (rule 10 governs).
Inside overlap at the zone = mark-room, fixed when the first boat reaches the zone. Don't take more than you're owed.
Opens the situation on the boat-length grid — scrub it and see exactly how the boats meet. Free, no account needed.
Rules cited
See the underlying rule: Rule 18 — Mark-Room at a Windward Mark.