Shape & trim
Leech line
A small adjustable cord inside the leech, used only to stop flutter.
A leech line is a thin cord sewn into the leech tape, led to a small cleat or clam near the clew or head. Its job is narrow and important: stop the leech from fluttering.
Flutter is not just noise. A vibrating edge breaks stitching, softens cloth and can destroy a sail surprisingly quickly. A light pull on the leech line supports the edge so it stops shaking.
The mistake is to overdo it. If the line is pulled too hard it hooks the leech to windward, closes the exit of the sail and makes a hard curl that is just as wrong as flutter. The right setting is the smallest tension that makes the edge quiet. If the line has to be bar-tight to work, the sail or the leech tape probably needs attention.