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A tell-tale or telltale is an indicator, signal, or sign that conveys the status of a situation, mechanism, or system.
In a vehicle, a tell-tale is an indicator on or near the dashboard to inform the driver that a system or device is operating, switched on, or that a problem has occurred with the vehicle.
In a nautical or sailing context a tell-tale, also known as a tell-tail, is a piece of yarn or fabric attached to a stay, any of several wires which hold a mast in place on a sailboat. they are used in pairs, on each side of the jib; there will be one tell-tail on the port stay and one on a starboard stay.
Tell-tales can also be attached to a sail, used as a guide when trimming (adjusting) a sail. 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. The solution is to bear away from the wind or sheet in. On the jib there may be 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-tail should stream aft when on a beat to windward. If one tell-tail begins to spiral, it is indicating the sail has detached air flow on that side. To correct this the sail needs to move towards the opposite side. "Tiller to tatteling tail" is a good phrase to remember which direction to push the tiller when the tail is spiraling. Alternatively, the sail itself can be sheeted in or out towards the tell-tale which is not streaming.
A tell-tale compass or repeating compass is a special compass installed in the ceiling of a cabin, and can be read from below or above deck. 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." 
The Phoenix spacecraft contains a tell-tale, developed by the University of Aarhus in Denmark, as part of its Meteorological Station. It is a small tube that is deflected by the martian wind. The science payload’s stereo camera recorded images of the tell-tale that are used to determine wind direction and speed.
More generally, in space systems a tell-tale is a single-bit status indicator that is included in telemetry or is used within the spacecraft's on-board software to signal conditions that must be tracked or acted upon, especially when the status changes. A tell-tale may continually change as the status it is tracking changes, or, it may change once upon change of status and then remain at that value until deliberately cleared. The former type of tell-tale is known as a "sticky-bit" because its value "sticks", that is, remains constant, once it has been set.
A tell-tale, also known as a bridge warning, is a series of ropes suspended over the tracks to give warning to a person on the roof of the train that the train is approaching a low-clearance obstacle, such as a tunnel or a bridge. A standard tell-tale design had ropes on 3-inch centers for a width of 8 feet over the track, the bottom of the ropes 6" lower than the height of the obstruction, and placed at least 100 feet before the obstruction.
Tell-tales in the above sense are also occasionally used to warn trucks and other tall vehicles of low clearance bridges on roads and highways. In this context, chains are used instead of ropes, and it is frequently the sound of the chains knocking against the truck that alerts the driver of trouble.
Formal language theory
In formal language theory in theoretical computer science, a tell-tale is a string of characters that occurs only within one language within a group of languages. A (human or machine) reader can be completely certain which language he/she/it is reading when encountering a tell-tale. In this sense, a tell tale is a dead giveaway 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.
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- "Sailing to the Telltales". UK-Halsey's Encyclopedia of Sails (Chapter 5). UKHalsey.com. Retrieved 2013-01-12.
- Melville, Herman (1992). "Chapter 51". Moby-Dick, or, The Whale. Illustrated by Rockwell Kent. Modern Library. p. 342. ISBN 0679600108.
- Mars Simulation Laboratory, University of Aarhus, Denmark, The Telltale project
- Slashdot 27may2008, Mars Probe Brings the "Weather Rock" New Respect
- Nasa Press Kit/May 2008 (ed.), Phoenix Landing Mission to the Martian Polar North (PDF), NASA
- Fundamentals of Space Sstems, Vincent L. Pisacane, editor. Oxford University Press, 2005. Page 629.
- Bridge Warning, Maintenance of Way Cyclopedia, Simmons Boardman and Co., 1921; page 246. Note: the equivalence of tell-tale and bridge warning is stated on page 278.
- Lyle Muller, Even chains can't help on Iowa Avenue, Cedar Rapids Gazette, May 4, 1997.
- Parkin Keith, British Rail Mark 1 Coaches 1991
- Dana Angluin (1980). "Inductive Inference of Formal Languages from Positive Data" (PDF). Information and Control. 45: 117–135. doi:10.1016/s0019-9958(80)90285-5. (link: citeseer.ist.psu).; here: Sect.3, p.120