On most sailboats, telltales are positioned on both sides of the leading edge of the jib (called the luff). These small strips of yarn or ribbon show how the air is flowing past the luff and can indicate when you need to trim the sail.
At the best sail trim, air flows smoothly past the luff on both sides of the sail. The telltales then stream back horizontally on both sides of the sail, as you can see in this photo. The red telltale is on the near side of the jib (to port), and the two green telltales are showing through from the other side of the sail (starboard).
This sail is in good trim because the telltales on both sides are streaming straight back. With good air flow on both sides, the sail’s shape generates power.
