Tell-tale (sailing)

On the mainsail tell-tales may be placed on the leech (aft edge) and when trimmed properly should be streaming backwards while on a beat (upwind).

When placed on the luff (forward or mast edge of the mainsail) they are used to indicate that the sail is luffing or coming head to wind.

As a general guide, the windward tell-tale should stream aft (backwards) with an occasional lift, the leeward front tell-tale should stream aft when on a beat to windward.

If one tell-tale begins to spiral, it is indicating the sail has detached air flow on that side.

According to Moby-Dick, a tell-tale refers to the cabin-compass, "because without going to the compass at the helm, the captain, while below, can inform himself of the course of the ship.