An unopened letter, which was mailed in 1697 but never delivered, was read by researchers who developed a way to essentially "unfold" sealed letter packets without ever having to crack the seal.

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

The "virtual unfolding" of the letter paves the way to a new line of historical research into the centuries-old tradition of letter locking. Experts say that the procedure used to show the text of the letter, which involves a form of imaging called X-ray micro-tomography, may also have applications in healthcare and engineering.

Letter locking 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 letter locked."

After finding such letters, Dambrogio coined the term "letter locking" while a fellow at the Vatican Secret Archives in 2000. Vatican letters dated back to the 15th and 16th centuries and featured odd slits and corners that had been cut off. Dambrogio discovered that the letters had originally been folded ingeniously, effectively "locked" by slipping a piece of paper through a slit and then covering it with wax. It would not have been possible to open the letter without tearing the slice of paper-proof that the letter had been tampered with.

Since then, Dambrogio has been practicing the art of letter locking, also making her own versions to showcase various techniques. The method dates back to the 13th century-at least in Western history-and several various folding and lock methods have evolved over the years. 

Queen Elizabeth I, Machiavelli, Galileo Galilei, and Marie Antoinette are among the prominent characters known to have used letter locking for their correspondence.

Researchers conclude the virtual unfolding would have an impact "far beyond," since so many collections around the world hold unopened letters.

For example, the letter from 1587 from Mary, Queen of Scots, to her brother-in-law, King Henry III of France, was locked using the so-called "butterfly lock"-only one of the hundreds of locking methods that Dambrogio had compiled into a letter lock dictionary. Other methods involve a simple triangular fold-and-tuck and an innovative system known as the "dagger-trap," which integrates a booby-trap disguised as another, simpler form of letter lock.

Many unopened letters are awaiting further study, including hundreds in the Prize Papers, a compilation of mail and other materials seized by Britain from enemy ships from the 17th to the 19th centuries.