Ariadne's Thread: According to myth, Ariadne gave Theseus a thread with which to find his way out of the Labyrinth. Today the term is used to describe navigation in difficult environments.

Ariadne was the daughter of King Minos of Crete, who attacked Athens after his son was murdered there. The Athenians submitted and had to sacrifice 14 youths to the Minotaur in his labyrinth every year. She fell in love with Theseus, a young man who volunteered to come and kill the Minotaur, and helped him by giving him a magic sword and a ball of thread so that he could easily find his way out. (Encyclopedia Mythica). Ariadne's story is told, among others, by Gaius Iulius Hyginus in his Fabulae (#42)

Other Links:
Bulfinch's Mythology
Encyclopedia Mythica

Snippets:
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Encyclopedia Britannica 1911 Edition
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