Rare 'locked' letter sealed 300 years ago is finally opened virtually Three hundred years ago, before envelopes, passwords and security codes, writers often struggled to keep thoughts, cares and dreams expressed in their letters private. One popular way was to use a technique called letter locking -- intricately folding a flat sheet of paper to become its own envelope. This security strategy presented a challenge when 577 locked letters delivered to The Hague in the Netherlands between 1689 and 1706 were found in a trunk of undelivered mail. The letters had never reached their final recipients, and conservationists didn't want to open and damage them. Instead, a team has found a way to read one of the letters without breaking its seal or unfolding it in any way. Using a highly sensitive X-ray scanner and computer algorithms, researchers virtually unfolded the unopened letter.