Page "Sail" Paragraph 25

Sail are often equipped with lightweight tapes or strands ( tell-tales ) to indicate the airflow in their area.

They may be on both sides near the leading edges of the sail, or at the trailing edge of the sail.

Horizontal strips sewn into fore-and-aft sails and V-shaped markings on spinnakers assist with judging their shape from on deck.

These may even glow in the dark, using luminous tapes.