This is the limit on "luffing rights". If you come from clear astern and establish an overlap to leeward of a boat on the same tack within two of your own hull lengths, you may not sail higher than your proper course while that overlap lasts and you stay within two lengths on the same tack.
Proper course is the course you would sail to get round the course fastest if the other boat were not there. You can still sail it, and if your proper course is up, you can go up. You just cannot luff above it to slow the other boat or push her away.
The limit ends when the overlap is broken, when the boats are more than two of your hull lengths apart, or when either boat changes tack. The one exception while it lasts: you may sail above your proper course if doing so takes you promptly astern of the other boat.
Common situations
Downwind leg, boat rolling from astern to leeward
A boat surfs up to leeward of another from astern. She may sail her proper course (often a little higher for speed) but may not luff above it to stop the windward boat. If she wants to luff, she must first drop behind and come back to windward, or get more than two lengths away to leeward and approach from there.
Overlap from a tack
If the overlap begins because the other boat tacked or gybed, the leeward boat did not come from clear astern and rule 17 does not apply to her. After the 2025 change, an overlap obtained while the windward boat is still tacking is no longer exempt either.
Common misreadings
"A boat with rule 17 on her has to hold her course"
She can sail her proper course, which may change with the wind. She just cannot go above it.
Changed in 2025
The 2021–2024 exception for an overlap that began while the windward boat was tacking has been deleted.