Two boats are beating upwind on opposite tacks — how do I tell in three seconds who keeps clear?
Read the booms. A boat is on the tack, port or starboard, that matches her windward side — so boom out to port means she is on starboard, boom out to starboard means she is on port. Then Rule 10 is simple: on opposite tacks the port-tack boat keeps clear. Who is ahead, who is faster and who shouted first decide nothing — and keeping clear has to be early enough that the starboard boat never has to change course.
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