Since the dawn of written communication, individuals have discovered methods to ensure their messages remain confidential. Prior to the creation of the gummed envelope in 1830, safeguarding correspondence required letterlocking, a clever technique involving the folding of a flat piece of paper to transform it into its own envelope, frequently utilizing a mix of folds, tucks, cuts, or adhesives like sealing wax. Authors from Erasmus to Catherine de’ Medici to Emily Dickinson utilized these methods, which Jana Dambrogio, the MIT Libraries’ Thomas F. Peterson (1957) Conservator, has termed “letterlocking.”

“The exploration of letterlocking deliberately connects the humanities and sciences,” explains Dambrogio, who initially became fascinated by the method as a fellow in the conservation studio of the Vatican Apostolic Archives, where she encountered examples dating back to the 15th and 16th centuries. “It incorporates insights from not only conservators and historians but also engineers, imaging specialists, and scientists.”

Now, the intricate history of this age-old document security technology is the focus of a new publication, “Letterlocking: The Hidden History of the Letter,” released by the MIT Press and co-authored with Daniel Starza Smith, a lecturer in early modern English literature at King’s College London. Dambrogio and Smith have been at the forefront of letterlocking research for the past decade, collaborating with a global and interdisciplinary group of specialists, the Unlocking History Research Group.

Featuring over 300 images and illustrations, “Letterlocking” delves into the historical context of the practice through authentic examples sourced globally. It includes a glossary of 60 technical terms and concepts, systems the authors crafted while examining over 250,000 historical letters. The objective of the book is to serve as a catalyst for new findings, whether by providing fresh insights into history or encouraging technological progress.

While engaging with the Brienne Collection — a 17th-century postal trunk filled with undelivered letters — the Unlocking History Research Group aimed to investigate preserved examples of locked letters without damaging them in the process. This prompted advancements in conservation, radiology, and computational algorithms. In 2020, the team collaborated with researchers from the MIT Computer Science and Artificial Intelligence Laboratory (CSAIL), Amanda Ghassaei SM ’17, and Holly Jackson ’22, to create new algorithms that could virtually decipher an unopened letter, publishing their findings in Nature Communications in 2021.

“Letterlocking” additionally provides a thorough guide for crafting one’s own locked letters. “The best way to begin exploring letterlocking is to construct some models,” advises Dambrogio. “Experience the shape and weight; notice how easily it can be concealed or how difficult it is to open without detection. We invite individuals to investigate and broaden this emerging field of study through ‘mind and hand.’”