The arms race between governments intent on surveillance and governed persons intent on not being surveilled goes back a long way. In a new piece at Atlas Obscura, Abigail Cain observes that “letters in the 1600s didn’t look exactly like letters today. Mass‐​produced envelopes weren’t invented until the 1830s, meaning that most 17th‐​century letter writers folded their correspondence in such a way that it became its own envelope—a process [MIT conservator and researcher Jana] Dambrogio had dubbed ‘letterlocking.’ Letterlocks could be simple, just a series of quick folds without any sort of adhesive. But they could also be incredibly complex, even booby‐​trapped to reveal evidence of tampering.”

Letterlocking might foil the curiosity of prying family members, but another objective was to frustrate “so‐​called ‘Black Chambers’—secret rooms attached to post offices and staffed by intelligence units, where mail was opened, copied, resealed, and sent on its way, with the writer and recipient none the wiser.” This helps explain a puzzlement that espionage expert and Leiden University senior lecturer Nadine Akkerman ran into during research into the Black Chambers: “a document by Samuel Morland, a British spymaster, in which he bragged about his talent for opening and resealing letters. ‘Wait,’ she thought to herself. ‘Can’t we all do that?’”

There was an incentive, of course, for letterlockers’ skills to improve in tandem with those of official snoops. “Just imagine that you throw 1,000 origami birds on a pile and have to refold them quickly to make sure that the recipient doesn’t know that you’ve actually read that letter,” Akkerman tells Cain. “Of course, that is an art.”

Letterlocking has drawn new interest from archivists, especially following the coming to light of a Dutch collection that included 577 unopened letters sent between 1689 and 1706. These days, researchers can deploy an extremely powerful scanner that can identify the ink deposits within such a letter, and thus permit its reading, without unlocking it. Samuel Morland would probably have been keen to get his hands on such a technology.

The article is fascinating throughout, and a related piece offers a simple introduction to how to lock your own letters.