Port-tack layline trap
The situation
Approaching the windward mark, Finn comes in on the port-tack layline and meets a starboard-tack boat also laying the mark. Finn calls for mark-room because he's the inside boat.
The question
Does Finn get mark-room from the starboard boat?
The ruling
No. Rule 18 does not apply between boats on opposite tacks on a beat to windward (rule 18.1(a)), so there is no mark-room between them — it's a straight port/starboard situation under rule 10. Finn (port) must keep clear of the starboard boat, by ducking or tacking (and rule 13 applies if he tacks). The port-tack layline approach into a fleet is a classic trap: it looks like an inside overlap but the mark-room rule simply isn't switched on.
Opposite tacks on the beat = no mark-room. The port boat keeps clear under rule 10, mark or no mark.
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 18.1 — Port-Tack Layline at the Windward Mark.