During a tack you have no rights under rule 10, rule 11 or rule 12. The keep-clear obligation starts when your bow passes head to wind and ends when the boat is on a close-hauled course on the new tack. The close-hauled heading counts, not whether the sails are trimmed or the boat is up to speed.
Once she reaches close-hauled the tacking boat may be the right-of-way boat immediately, but she has only just acquired right of way, so rule 15 makes her give the other boat room to keep clear.
If two boats are both tacking at the same time, the one on the other's port side, or the one astern, keeps clear.
Common situations
Tacking under a starboard boat
A port boat tacks to leeward and ahead of a starboard boat ("lee-bowing"). Until the tacking boat reaches close-hauled on starboard she must keep clear. Once there, she is leeward or clear ahead, and the other boat must keep clear, but only after being given room to do so under rule 15. A tack that finishes inside a boat length, forcing the other boat to luff hard straight away, usually breaks rule 13 or rule 15.
Slam dunk
When a starboard boat tacks on top of a port boat that is ducking her, the port boat may get an overlap to leeward while the starboard boat is still tacking. Since 2025 nothing exempts that overlap from rule 17, so the boat that came from astern cannot sail above her proper course afterwards.
Common misreadings
"The tack ends when the sails fill"
It ends at the close-hauled heading. A boat that has turned to close-hauled but is still slow is no longer tacking. Appendix B changes this for windsurfers, where the tack ends when the sail has filled.