More than 600 years ago, someone intricately folded, sealed and posted a letter that was never delivered. Now, scientists have digitally "unfolded" this and other similarly locked letters found in a 17th-century trunk in The Hague, using X-rays.

For centuries prior to the invention of sealed envelopes, sensitive correspondence was protected from prying eyes through complex folding techniques called "letterlocking," which transformed a letter into its own secure envelope. However, locked letters that survive to the present are fragile and can be opened physically only by slicing them to pieces.

The new X-ray method offers researchers a non-invasive alternative, maintaining a letterpacket's original folded shape. For the first time, scientists applied this method to "locked" letters from the Renaissance period, kept in a trunk that had been in the collection of the Dutch postal museum in The Hague, The Netherlands, since 1926.

Related: Photos: Treasure trove of unopened 17th-century letters

The trunk's contents include more than 3,100 undelivered letters, of which 577 were unopened and letterlocked. Known as the Brienne Collection, the letters were written in Dutch, English, French, Italian, Latin and Spanish. For unknown reasons, once the missives reached The Hague they were never delivered to their intended recipients, and were instead kept by a postmaster named Simon de Brienne, Live Science previously reported.

Locked letters used different mechanisms to stay securely closed, including folds and rolls; slits and holes; tucks and adhesives; and a variety of cleverly constructed locks, according to a study published online March 2 in the journal Nature Communications.

To penetrate the layers of folded paper, the study authors used an X‐ray microtomography scanner engineered in the dental research labs at Queen Mary University of London (QMU). Researchers designed the scanner to be exceptionally sensitive so that it could map the mineral content of teeth, "which is invaluable in dental research," study co-author Graham Davis, a QMU professor of 3D X-ray imaging, said in a statement.

"But this high sensitivity has also made it possible to resolve certain types of ink in paper and parchment," Davis added.

A seventeenth-century trunk that was bequeathed to the Dutch postal museum in the Hague contained thousands of letters that were sent from all over Europe and were never delivered. (Image credit: Courtesy of the Unlocking History Research Group archive)RELATED CONTENT

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"The rest of the team were then able to take our scan images and turn them into letters they could open virtually and read for the first time in over 300 years," study co-author David Mills, an X-ray microtomography facilities manager at QMU, said in the statement.

From the scans, the team built 3D digital reconstructions of the letters, and then created a computational algorithm that deciphered the sophisticated folding techniques, crease by crease, opening the letters virtually "while preserving letterlocking evidence," according to the study.

"Virtual unfolding" enabled researchers to read the contents of sealed letterpackets from 17th-century Europe without physically opening them. (Image credit: Courtesy of the Unlocking History Research Group archive)The scientists digitally opened four letters using this groundbreaking method, deciphering the contents of one letter, DB-1627. Penned on July 31, 1697, it was written by a man named Jacques Sennacques to his cousin Pierre Le Pers, who lived in The Hague. Sennacques, a legal professional in Lille, France, requested an official death certificate for a relative named Daniel Le Pers, "perhaps due to a question of inheritance," the scientists wrote.

"His request issued, Sennacques then spends the rest of the letter asking for news of the family and commending his cousin to the graces of God," the authors wrote. "We do not know exactly why Le Pers did not receive Sennacques' letter, but given the itinerancy of merchants, it is likely that Le Pers had moved on." Tens of thousands of such sealed documents can now be unfolded and read virtually, the researchers reported.

"This algorithm takes us right into the heart of a locked letter," the research team said in the statement. "Using virtual unfolding to read an intimate story that has never seen the light of day — and never even reached its recipient — is truly extraordinary."

Originally published on Live Science.