Telltales are usually just thin strips of cloth or light cord that are stuck on the sails at critical points to show you how the air is flowing across the surface. A telltale that is flying at 90 degrees usually means the sail is well trimmed. If the windward (the side of the sail the wind is blowing into) telltales are riding upwards you need to pull the sail in a little (sheet in) and if the tellates on the leeward side (opposite side to windward!) are riding upwards you need to let the sail out a little (sail out).
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