Late at night on 8 February 1587, an imprisoned Mary Queen of Scots composed her last ever letter to her brother-in-law. “Tonight, after dinner, I have been advised of my sentence: I am to be executed like a criminal at eight in the morning,” she wrote. “The Catholic faith and the assertion of my God-given right to the English crown are the two issues on which I am condemned.” With a sad acceptance of her fate, she asked him to take care of her affairs and pay her servants, wishing him “good health and a long and happy life”.

After Mary had finished writing, she then began to fold up the letter to secure its contents. She didn’t want her captors snooping – and particularly not her cousin Queen Elizabeth I. However, envelopes were not used in the 1500s – not least because paper was expensive – and there was no trustworthy postal service at the time.

Instead, Mary cut a thin strip from the paper margin, before folding up her message into a small rectangle. After poking the knife through the rectangle to make a hole, she then fed the strip through, looping it and tightening it a few times, creating a “spiral lock”. No wax or adhesive was required, but crucially, if someone tried to sneak a look, they would have to rip through the strip, so her brother-in-law would know the message had been intercepted.

Watch a reconstruction of how Mary did it:

How Mary Queen of Scots locked her last letter (Credit: Unlocking History Research Group)

• Watch this and other letterlocking techniques on the Unlocking History Research Group’s YouTube page

Mary Queen of Scots was far from the only person who was skilled in the art of “letterlocking” – the technique became common throughout Europe during the Late Middle Ages (1250-1500) and Early Modern periods (1500-1815). By folding and cutting letters in various clever patterns, people attempted to hide their correspondence from unwanted readers, and the “locks” came in myriad types.

“This isn’t something special that people do on special occasions. This is how you send a letter before the envelope is invented,” explains Daniel Starza Smith, a lecturer in Early Modern English literature at King’s College London. “So, if it’s a business letter, if it’s a love letter, if it’s a spy letter, if it’s a diplomatic letter, they’re all using letterlocking. So it’s not something confined to experts, royalty or spy masters. Anyone who is capable of sending a letter is using letterlocking.”

But in the present day, we’re only beginning to understand the technique’s importance in history. In recent years, a whole taxonomy of apparently forgotten letterlocking tricks have been uncovered.

So how does letterlocking work, and is it possible to try it yourself?

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The modern study of letterlocking began when the conservator Jana Dambrogio was leafing through a cache of documents in the Vatican Secret Archives in Italy. In the early 2000s, she had been the first woman from outside the archive’s conservation laboratory who was allowed to work there. On her first day, she was offered the chance to work on the Fondo Veneto Sezione II, a cache of maps, letters, legal and accounting documents from the late 1500s, many of which had not been repaired.

As she completed her conservation work, Dambrogio didn’t always follow the writing, since it was often in old Italian dialects, but she did notice cuts, creases and folds in the paper. While it could be mistaken for damage to the untrained eye, she realised it was evidence of letterlocking. So, she methodically described what she observed.

A rare unopened example of a locked letter, with red crayon marking the postage cost (Credit: Unlocking History Research Group)

When Dambrogio returned to the US – she’s now based at MIT in Massachusetts – those notes and models of the original documents would come in useful. “There are probably thousands of letters of the Vatican, but this handful that I modelled started to help us build the language of letterlocking,” she explains.

After connecting with Starza Smith, she and her colleagues began to seek out more examples of letterlocking wherever they could find them, often hidden away in old archives and museum collections.

A few years ago, they came across one particularly rich bounty: a whole trunk full of 2,600 letters from 17th-Century Europe which had gone undelivered – 577 of which were unopened.

The trunk holding the Brienne Collection, a cache of undelivered letters from the 17th Century (Credit: Sound and Vision The Hague, The Netherlands)

The collection had been kept by a married postmaster and postmistress called Simon Brienne and Marie Germain, who lived in the Netherlands. In the 1600s, the recipients of letters had to pay to receive them, so for various reasons – poverty, relocation or death – letters often failed to reach their destination. Many had the words