Shape & trim
Telltale
A small yarn or ribbon that shows how air is flowing over the sail.
DutchVerklikkerour Dutch loft →
SpanishCatavientosour Spanish loft →
A telltale is a short piece of yarn, ribbon or light cloth fixed to the sail so you can see the airflow. On a headsail you normally have pairs on both sides near the luff. On a mainsail you often see them streaming from the leech.
When both windward and leeward telltales stream aft, the sail is meeting the wind cleanly. If the windward one lifts, you are sailing too high or the sail is too eased. If the leeward one stalls, the sail is too tight or the boat is too low.
Leech telltales read the exit. A main with the top telltale hidden behind a tight leech is usually overtrimmed; one with every telltale flying all the time may be too open. They are simple, cheap and more honest than guessing by feel.