clew

The "ball of thread" meaning of clew (from Middle English clewe and ultimately from Old English cliewen) has been with us since before the 12th century. In Greek mythology, Ariadne gave a ball of thread to Theseus so that he could use it to find his way out of her father's labyrinth. This, and similar tales, gave rise to the use of clew for anything that could guide a person through a difficult place. This use led, in turn, to the meaning "a piece of evidence that leads one toward the solution of a problem." Today, the variant spelling clue, which appeared in the 17th century, is the more common spelling for the "evidence" sense, but you'll find clew in some famous works of literature. Also, clew is the only choice for the sailing senses.