On the Same Tack, Not Overlapped
Rule 12 - ON THE SAME TACK, NOT OVERLAPPED
When boats are on the same tack and not overlapped, a boat clear astern shall keep clear of a boat clear ahead.
It's easy to think "the boat coming up from behind gives way to the boat that's ahead", but the thing to remember here is that this rule only applies to boats on the same tack.
If the Clear Astern boat is on Starboard while the Clear Ahead boat is on Port, that is a Rule 10 scenario, not a Rule 12 scenario.
A boat keeps clear of a right-of-way boat
(a) if the right-of-way boat can sail her course with no need to take avoiding action and,
(b) when the boats are overlapped, if the right-of-way boat can also change course in both directions without immediately making contact.
Leeward and Windward
A boat’s leeward side is the side that is or, when she is head to wind, was away from the wind.
However, when sailing by the lee or directly downwind, her leeward side is the side on which her mainsail lies. The other side is her windward side.
When two boats on the same tack overlap, the one on the leeward side of the other is the leeward boat. The other is the windward boat.
Tack, Starboard or Port
A boat is on the tack, starboard or port, corresponding to her windward side.
Clear Astern and Clear Ahead; Overlap
One boat is clear astern of another when her hull and equipment in normal position are behind a line abeam from the aftermost point of the other boat’s hull and equipment in normal position. The other boat is clear ahead. They overlap when neither is clear astern.
However, they also overlap when a boat between them overlaps both.
These terms always apply to boats on the same tack.
They apply to boats on opposite tacks only when rule 18 applies between them or when both boats are sailing more than ninety degrees from the true wind.
Example: Blue (Clear Astern) must keep clear of Yellow (Clear Ahead)
Blue (Clear Astern) catching up to Yellow (Clear Ahead), both on the Same Tack
Options for Blue (Clear Astern):
Don't run into the back of Yellow!