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Researchers virtually open and read sealed historic letters

History unfolds as an interdisciplinary research team uses computational tools to examine the contents of “locked” letters.
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historic letter shown in various stages of unfolding
Using computational tools, a research team was able to virtually unfold an unopened letter from 1697.
Image: Courtesy of MIT Libraries
Photo of a historic letter with French handwriting and the number 10 in red
Researchers were able to read the contents of a letter from Jacques Sennacques to his cousin Pierre Le Pers, a French merchant in The Hague.
Black and white scans of a flattened letter with faint handwriting on it
The virtual unfolding technology generates 2D and 3D reconstructions of the letter.

An international team of scholars has read an unopened letter from early modern Europe — without breaking its seal or damaging it in any way — using an automated computational flattening algorithm. The team, including MIT Libraries and Computer Science and Artificial Intelligence Laboratory (CSAIL) researchers and an MIT student and alumna, published their findings today in a Nature Communications article titled, “Unlocking history through automated virtual unfolding of sealed documents imaged by X-ray microtomography.” 

The senders of these letters had closed them using “letterlocking,” the historical process of folding and securing a flat sheet of paper to become its own envelope. Jana Dambrogio, the Thomas F. Peterson Conservator at MIT Libraries, developed letterlocking as a field of study with Daniel Starza Smith, a lecturer in early modern English literature at King’s College London, and the Unlocking History research team. Since the papers’ folds, tucks, and slits are themselves valuable evidence for historians and conservators, being able to examine the letters’ contents without irrevocably damaging them is a major advancement in the study of historic documents.

“Letterlocking was an everyday activity for centuries, across cultures, borders, and social classes,” explains Dambrogio. “It plays an integral role in the history of secrecy systems as the missing link between physical communications security techniques from the ancient world and modern digital cryptography. This research takes us right into the heart of a locked letter.”

This breakthrough technique was the result of an international and interdisciplinary collaboration between conservators, historians, engineers, imaging experts, and other scholars. "The power of collaboration is that we can combine our different interests and tools to solve bigger problems," says Martin Demaine, artist-in-residence in MIT’s Department of Electrical Engineering and Computer Science (EECS) and a member of the research team. 

The algorithm that makes the virtual unfolding possible was developed by Amanda Ghassaei SM ’17 and Holly Jackson, an undergraduate student in electrical engineering and computer science and a participant in MIT’s Undergraduate Research Opportunity Program (UROP), both working at the Center for Bits and Atoms. The virtual unfolding code is openly available on GitHub

“When we got back the first scans of the letter packets, we were instantly hooked,” says Ghassaei. “Sealed letters are very intriguing objects, and these examples are particularly interesting because of the special attention paid to securing them shut.”

Secrets revealed

“We’re X-raying history,” says team member David Mills, X-ray microtomography facilities manager at Queen Mary University of London. Mills, together with Graham Davis, professor of 3D X-ray imaging at Queen Mary, used machines specially designed for use in dentistry to scan unopened “locked” letters from the 17th century. This resulted in high-resolution volumetric scans, produced by high-contrast time delay integration X-ray microtomography.

“Who would have thought that a scanner designed to look at teeth would take us so far?” says Davis.

Computational flattening algorithms were then applied to the scans of the letters. This has been done successfully before with scrolls, books, and documents with one or two folds. The intricate folding configurations of the “locked” letters, however, posed unique technical challenges.

“The algorithm ends up doing an impressive job at separating the layers of paper, despite their extreme thinness and tiny gaps between them, sometimes less than the resolution of the scan,” says Erik Demaine, professor of computer science at MIT and an expert in computational origami. “We weren't sure it would be possible."

The team’s approach utilizes a fully 3D geometric analysis that requires no prior information about the number or types of folds or letters in a letter packet. The virtual unfolding generates 2D and 3D reconstructions of the letters in both folded and flat states, plus images of the letters’ writing surfaces and crease patterns. 

“One of coolest technical contributions of the work is a technique that explores the folded and flattened representations of a letter simultaneously,” says Holly Jackson. “Our new technology enables conservators to preserve a letter’s internal engineering, while still giving historians insight into the lives of the senders and recipients.”

This virtual unfolding technique was used to reveal the contents of a letter dated July 31, 1697. It contains a request from Jacques Sennacques to his cousin Pierre Le Pers, a French merchant in The Hague, for a certified copy of a death notice of one Daniel Le Pers. The letter comes from the Brienne Collection, a European postmaster’s trunk preserving 300-year-old undelivered mail, which has provided a rare opportunity for researchers to study sealed locked letters. 

“The trunk is a unique time capsule,” says David van der Linden, assistant professor in early modern history, Radboud University Nijmegen. “It preserves precious insights into the lives of thousands of people from all levels of society, including itinerant musicians, diplomats, and religious refugees. As historians, we regularly explore the lives of people who lived in the past, but to read an intimate story that has never seen the light of day — and never even reached its recipient — is truly extraordinary.”

Advancing a new field

In the Nature Communications article, the team also unveils the first systematization of letterlocking techniques. After studying 250,000 historical letters, they devised a chart of categories and formats that assigns letter examples a security score. Understanding these security techniques of historical correspondence means archival collections can be conserved in ways that protect small but important material details, such as slits, locks, and creases. 

“Sometimes the past resists scrutiny,” explains Daniel Starza Smith. “We could simply have cut these letters open, but instead we took the time to study them for their hidden, secret, and inaccessible qualities. We’ve learned that letters can be a lot more revealing when they are left unopened.”

The research team hopes to make a study collection of letterlocking examples available to scholars and students from a range of disciplines. The virtual unfolding algorithm could also have broad applications: Because it can handle flat, curved, and sharply folded materials, it can be used on many types of historical texts, including letters, scrolls, and books. 

“What we have achieved is more than simply opening the unopenable, and reading the unreadable,” says Nadine Akkerman, reader in early modern English literature at Leiden University. “We have shown how truly interdisciplinary work breaks down boundaries to investigate what neither humanities nor the sciences can hope to understand alone.” 

Computational tools promise to accelerate research on letterlocking as well as reveal new historical evidence. Thanks to this research, adds Rebekah Ahrendt, associate professor of musicology at Utrecht University, “we can now imagine new affective histories that physically connect the past and the present, the human and the nonhuman, the tangible and the digital.”

The research team includes Jana Dambrogio, Thomas F. Peterson Conservator, MIT Libraries; Amanda Ghassaei, research engineer at Adobe Research; Daniel Starza Smith, lecturer in early modern English literature at King’s College London; Holly Jackson, undergraduate student at MIT; Erik Demaine, professor in EECS; Martin Demaine, robotics engineer in CSAIL and Angelika and Barton Weller Artist-in-Residence in EECS; Graham Davis and David Mills, Queen Mary University of London's Institute of Dentistry; Rebekah Ahrendt, associate professor of musicology at Utrecht University; Nadine Akkerman, reader in early modern English literature at Leiden University; and David van der Linden, assistant professor in early modern history at Radboud University Nijmegen.

This research was supported in part by grants from the Seaver Foundation, the Delmas Foundation, the British Academy, and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek.

Press Mentions

New York Times

An international team of scholars, including MIT researchers, has published a new study exploring the history and use of letterlocking, reports William J. Broad for The New York Times. The researchers note that they hope their work prompts “novel kinds of archival research, and allows even very well-known artefacts to be examined anew.”


Researchers from MIT and other institutions have successfully uncovered the letterlocking technique that Mary, Queen of Scots, used to seal her final letter, reports NPR’s Tien Le. The spiral lock requires more than 30 steps and involves cutting out a ‘lock,’ often resembling a dagger or sword, out of the blank margin of the letter,” writes Le. “The lock acts as a needle and is sewn through the letter after folding it.”

The Guardian

An international team of researchers has found that Mary, Queen of Scots, used a complicated paper-folding technique called letterlocking to conceal the contents of her final letter, reports Alison Flood for The Guardian. MIT Libraries Conservator Jana Dambrogio explains that working on Mary's last letter “and figuring out its unique spiral lock was thrilling as a researcher – and a real a-ha! moment in the study of letterlocking.”

The Economist

The Economist spotlights how MIT researchers created a virtual technique to decipher the contents of a letter that was sealed 300 years ago. The letter was originally sealed by its sender using the historical practice of securing correspondence called letterlocking. The new virtual technique “seems to hold plenty of promise for future research into a fascinating historical practice.”

New Scientist

Using X-ray imaging and algorithms, MIT researchers have been able to virtually open and read letters that been sealed for more than 300 years, writes Priti Parikh for New Scientist. “Studying folding and tucking patterns in historic letters allows us to understand technologies used to communicate,” says Jana Dambrogio, a conservator at the MIT Libraries.

The Guardian

Guardian reporter Alison Flood explores the new technique created by MIT researchers to virtually unseal an unopened letter written in 1697. The researchers, “worked with X-ray microtomography scans of the letter, which use X-rays to see inside the document, slice by slice, and create a 3D image,” writes Flood.

New York Times

Researchers from MIT and other institutions have developed a new virtual-reality technique that has allowed them to unearth the contents of letters written hundreds of years ago, without opening them, writes New York Times reporter William J. Broad. “The new technique could open a window into the long history of communications security,” writes Broad. “And by unlocking private intimacies, it could aid researchers studying stories concealed in fragile pages found in archives all over the world.”


A new imaging technique created by researchers from MIT and other institutions has been used to shed light on the contents of an unopened letter from 1697, writes Matt Simon for Wired. “With fancy letterlocking techniques, you will forcibly rip some part of the paper, and then that will become detectable,” says Prof. Erik Demaine, of the method used to seal the letter.

The Wall Street Journal

Researchers from MIT and other institutions have used algorithms and an X-ray scanner to decipher the secrets inside a letter that has been sealed since 1697, reports Sara Castellanos for The Wall Street Journal. “This is a dream come true in the field of conservation,” said Jana Dambrogio, the Thomas F. Peterson Conservator at MIT Libraries.


Jana Dambrogio, the Thomas F. Peterson Conservator at MIT Libraries, describes the new technique developed by researchers at MIT and other institutions that has allowed for the virtual exploration of a letter that has been sealed since 1697. “It's quite beautiful and it's thrilling that we can read it without tampering with the letter packet, leaving it to study as an unopened object,” says Dambrogio.

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