Telltales are lightweight ribbons, yarns, or fabric strips attached to sails to indicate airflow, acting as crucial, real-time feedback for sail trim and steering. Usually positioned near the luff (leading edge) of the jib and on the leech (trailing edge) of the mainsail, they help visualize laminar flow, revealing stalls, turbulence, or if the sail is over/undersheeted.

Key Telltale Characteristics & Usage:
Location: Placed in pairs (one on each side) on the headsail (windward/leeward) and sometimes on the mainsail, often at multiple heights.

Optimal Flow: Both telltales should stream aft smoothly; this means airflow is attached to the sail, producing maximum lift.

Windward Telltale Lifting (Up): Indicates the sail is under-sheeted (too loose) or the boat is pointed too high into the wind ("pinching").

Leeward Telltale Lifting (Up or Dancing): Indicates the sail is over-sheeted (too tight) or the boat is falling off the wind.

Materials: Frequently red and green (starboard/port) for easy identification, made from wool, nylon, or plastic, often attached using adhesive discs.