MIT reports some of its researchers have figured out how to use X rays to virtually unravel and even read centuries-old letters that were carefully folded and "locked" in the days before envelopes were invented and offered some measure of privacy.
Using X-ray devices designed for dentists, and with researchers at Queen Mary University in London, the researchers first would scan the letters, then use specialized software - written by an MIT graduate and an MIT undergrad - to analyze and "unfold" the letters and create "2D and 3D reconstructions of the letters in both folded and flat states, plus images of the letters’ writing surfaces and crease patterns."
Why not just rip open the paper and read the letters? Although the original recipients are long gone and so would not much care, historians of the eras from which the letters came would likely react harshly, since "the papers’ folds, tucks, and slits are themselves valuable evidence for historians and conservators."
More on letterlocking.