Overtaking to windward
The situation
On a reach, Blue catches Green from clear astern and goes to pass to windward, establishing a windward overlap.
The question
Who must keep clear, before and after the overlap forms?
The ruling
Blue must keep clear throughout. While clear astern she keeps clear under rule 12; once overlapped to windward she keeps clear under rule 11. Because Green acquired her right of way through Blue's own action of establishing the overlap, rule 15 does not require Green to give room — the keep-clear duty rests entirely with Blue.
An overtaking boat passing to windward keeps clear the whole time — both before and after the overlap.
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 12 — On the Same Tack, Not Overlapped.