Why do we say "X Marks The Spot"?

Background:

The letter “X” is extremely versatile. As stated in a article on the Word Stories WordPress blog, ”No other single letter really does polysemy like x: it can stand for a kiss, found from 1765; it can represent the horizontal axis on a graph; it designates films which are only appropriate for adults, found from the 1950s; it can represent a cross; it can indicate a mistake; or it can denote any unknown or unspecified thing, like in algebra.” In this case, an X is used to designate a specific location.

Most associate “X marks the spot” with old-time pirate maps where an X marked the location of buried treasure, and this is mostly due to Scottish author Robert Louis Stevenson’s use of the device in his adventure novel Treasure Island, published in 1883. In the story, the pirates are using a tattered map left by the notorious Captain Flint to find a cache of gold and jewels, indicated by an X on the parchment. Once they reach the location there is, in fact, an X on the ground in the exact place they’re to dig — X marks the spot — where (spoiler alert) they do find a stash of riches.

Using an X as an indication on a map predates Treasure Island, however. It’s thought that Xs were commonly found on maps of all sorts long before Stephenson’s novel, particularly navigational maps used by the British navy.

Another possible origin for the term “X marks the spot” stems from the British military. Executions were relatively common during the 19th century; during the Napoleonic Wars (1803-1815), Arthur Wellesley, the First Duke of Wellington (aka the Iron Duke) executed at least 112 of his men. Soldier or sailor could be put to death for desertion, mutiny, gross insubordination, and cowardice, among other crimes. This was often done by firing squad, and the practice was to pin a piece of paper with a black X on it over the condemned man’s heart as a target — hence, “X marks the spot.”

The phrase gained traction in the United States during the Gangster Era (approximately 1920-1938). When Prohibition was enacted, banning alcohol, some enterprising individuals found a way to make a lot of money by skirting the law. This led to the rise of criminal gangs, and as each sought to control more territory, violent confrontations ensued (think of shoot-outs with machine guns). As the press reported on the aftermath, the photos they published would omit dead bodies, indicating them with an X. “X marks the spot” consequently became a very popular idiom at the time. Interestingly, “the spot” became a euphemism for a dead body, leading to the popularization of another phrase: to put someone on the spot, meaning to place them in a difficult situation.

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