Sailing crafts
In a nautical or sailing context a tell-tale is a piece of cloth or fabric that is tied or attached to a stay, one of several wires, that holds a mast in place on a sailboat. Usually there will be one tell-tale on the port stay and one on a starboard stay. When the wind blows the cloth waves much as a pennant or flag would flutter and gives you the relative direction of the wind. This aids navigation so you can adjust your direction with the rudder or a sail jibe or other sail adjustment to modify your direction. Thus, in lay terms that means that a "tell-tale" is a reference indicator or a sign that clearly signals that something else is true or is about to happen. Tell-tales can also be attached to a sail, used as a guide when trimming (adjusting) a sail. On the mainsail tell-tales are on the leech (aft edge) and when trimmed properly should be streaming backwards. On the jib there are tell-tales on both sides of the luff of the sail. As a general guide, the windward tell-tale should stream aft (backwards) with an occasional lift, the leeward front tell-tale should stream aft.
Steam locomotive usage
In a steam locomotive, tell-tales are longitudinal holes drilled in the stays of the firebox to provide early warning of corrosion.
Linguistics
A tell-tale is a string of characters that occurs only within one language within a group of languages. A reader can be completely certain which language they are reading if he or she comes across a tell-tale. In this sense, a tell tale is a dead give away of what the language is. More formally, a tell-tale of a member L of some language class is a finite subset of L such that no other language containing the subset in the class is a proper subset of L. In other words, a tell-tale is a finite subset that makes a language being a minimal consistent one in the class. The term is used in the field of artificial intelligence and machine language learning as well as linguistics. See also, Language identification in the limit
