Between two boats on opposite tacks, upwind or downwind, before or after the start, unless a rule of Section C or D takes over.
The oldest rule on the water and still the one behind most collisions. Your tack is named for your windward side, which for a boat sailing normally is the side opposite the main boom. Starboard tack has the wind coming over the starboard side; port tack the other way.
The port-tack boat must keep clear. That is more than not hitting: the starboard boat must be able to hold her course without having to do anything to avoid you. If she has to luff, bear away or even hesitate because you are coming, you have not kept clear, whether or not anyone touches.
Starboard has rights but not licence. She must still avoid contact if she can (rule 14), and if she changes course she must give port room to keep clear (rule 16).
Common situations
Port crossing ahead upwind
Port may cross ahead if she clears starboard without starboard needing to change course. When it is close, starboard does not have to wait and see: if a reasonably competent helm on starboard would need to act to avoid contact, port has broken the rule even if starboard holds her course and port scrapes past.
Port ducking starboard
A port boat bearing away to pass astern is keeping clear. Starboard may not then change course to hunt her; after the starting signal, rule 16.2 limits exactly that.
Downwind, by the lee
When a boat sails by the lee her tack is set by where her mainsail lies, not by the apparent wind direction. A boat with the boom out to port is on starboard tack, even if she is sailing slightly by the lee.
Common misreadings
"We did not touch, so there is no foul"
Contact is not the test. A protest committee asks whether starboard needed to take avoiding action. A visible luff or bear-away by starboard at the moment of crossing is the usual evidence.
"Starboard has to hold her course"
No rule requires a right-of-way boat to hold a course. She may change course freely, but when she does she must give the keep-clear boat room to respond (rule 16.1).