This mark shows the scene of the crime, the hiding place of a treasure, or some other special location. Although this term dates only from the nineteenth century, the use of a cross or the letter X as a special indicator is surely much older. The OED’s earliest citation is from a letter by Maria Edgeworth in 1813: “The three crosses X mark the three places where we were let in.” The term often appeared in romantic pirate stories in which hidden treasure marked on a map figured, as in Robert Louis Stevenson’s Treasure Island (1883).