​The telltales, as the most important visual aid, reveal the dynamic flow of air around the sails that obtain from moment to moment. Simple pieces of yarn or plastic strips attached to the sails, telltales are about as low-tech as you can get, but they are also nearly infallible indicators of proper trim. Broadly speaking, when the telltales are streaming aft in unison, then the sails are properly trimmed. Some people say, "I don't use the telltales; i sail better by feel." Not possible. These same sailors tend to overtrim their sails because flat, tight sails tend to feel and look faster than loosely trimmed sails. Those simple strips of yarn feel the wind far more accurately than any sailor. 

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Since we can't see the wind flowing over the sail we use "telltales" attached to the sail in strategic positions to "tell" us what the wind is doing at that exact location. They are mounted on both sides of the sail to tell us what is happening to the airflow on either side.

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If the telltales are flying smoothly backwards then they are telling us that the air at that point is flowing smoothly without turbulence and thus the sail is set at its best setting. If the telltales are flying wildly it indicates that turbulent flow is happening due to less than optimal sail set and some trimming is needed.

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Thus they are sometimes referred to as woolies. On the port side of the sails, the telltales are red. On the starboard side of the sails the telltales are green. Many times the telltales are mounted on a see through mylar window. This way you can see both telltales at the same time. Telltales are also placed vertically up the sail so that you can see if the sail is set properly at that height.