An unopened letter that was mailed back in 1697 but never delivered has been read by researchers who have developed a way to virtually "unfold" sealed letter packets without having to actually break the seal.

The new technique, described in the journal Nature Communications, should allow historians to learn more about "letterlocking," the practice of using elaborate slits, folds, creases and tucks to turn a flat sheet of paper with a written message into a tamper-resistant package.

Such security measures were an everyday part of life for centuries. "The envelope as we know it, the gummed envelope, wasn't invented until the 1830s," says Jana Dambrogio, a conservator with the Massachusetts Institute of Technology Libraries in Cambridge, Mass. "And so before then, everyone letterlocked."

Letterlocking technologies varied greatly, with people folding and sealing their messages in hundreds of different ways. "They can make it look like it's less secure and have security hidden on the inside," says Dambrogio. "Or they can make something look very secure, but it actually isn't that secure on the inside, when you open the letter packet."