Covert operations: How European royals once shared their important secrets

They’d fold the letter, then cut a dangling strip, using that as an improvised thread to sew stitches that locked the letter and turned the flat writing paper into its own envelope. To get inside, a spy would have to snip the lock open, an act impossible to go undetected.

Catherine de’ Medici used the method in 1570 — a time she governed France while her ill son, King Charles IX, sat on its throne. Queen Elizabeth did so in 1573 as the sovereign ruler of England and Ireland. And Mary Queen of Scots used it in 1587 just hours before her long effort to unite Britain ended in her beheading. “These people knew more than one way to send a letter and they chose this one,” said Jana Dambrogio, lead author of a study that details Renaissance-era politicians’ use of the technique, and a conservator at the M.I.T. Libraries. “You had to be highly confident to make a spiral lock. If you made a mistake, you’d have to start all over, which could take hours of rewriting and restitching. It’s fascinating. They took great pains to build up their security.” Disclosure of the method’s wide use among European royalty is the latest venture of a group of scholars, centered at M.I.T., into a vanished art they call letterlocking — an early form of communications security that they’re busy resurrecting. Early last year, they reported their development of a virtual-reality technique that let them peer into locked letters without tearing them apart and damaging the historical record.

Now, in a detailed article that appeared last month in the Electronic British Library Journal, the scholars lay out their expanding universe of discoveries and questions. They showcase instances of spiral letterlocking among the queens and posit that the method “spread across European courts through royal correspondence.” Although the use of locked letters faded in the 1830s with the emergence of mass-produced envelopes and improved systems of mail delivery, it’s now seen as a fascinating precursor to the widespread encryption used globally in electronic communications.

In their recent paper, the authors use case studies of locked letters as well as graphic illustrations and detailed descriptions of the process to reveal what they’ve learned in two decades of study. The paper’s main objective is to help other scholars identify when the technique was used in historic letters that have already been opened, flattened and repaired in ways that leave few traces of their original state.

The authors say collections of libraries and archives often hold examples of letterlocking that are hidden in plain sight. Knowledge of the technique, they add, can be used to recover nuances of personal communication that, until now, have been lost to history. “We hope,” the authors write, that their finds prompt “novel kinds of archival research, and allow even very well-known artefacts to be examined anew.” The nine authors of the new paper, in addition to Dambrogio, include students at M.I.T. as well as scholars from King’s College London, the University of Glasgow and the British Library. The British Library has an ongoing exhibition that highlights some of the unlocked letters.

Broad is a science journalist with NYT©2022

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