Cuts & construction
Luff attachment
How the luff connects to the mast, stay or furler; the detail that must match the boat.
Luff attachment is the hardware and edge construction that connects the sail's luff to the boat. On a mainsail that might be a bolt rope in a mast groove, slides in a track, slugs, or batten cars. On a headsail it might be hanks, a foil tape, or a furling luff tape.
This is why a used sail can measure right and still be useless. The luff length, foot and leech may all fit the rig, but a bolt rope will not enter a slide track, and slides made for one mast section may not run in another. The sail is the right size and still cannot go up.
Attachment also affects handling. Slides keep a mainsail captive when lowered; a bolt rope is simpler but must feed cleanly. Full-batten sails usually need better hardware because batten pressure adds friction at the mast.