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...