A fascinating article worth the keep in our Historical Tidbit section of letter locking.
Mary Queen of Scots was far from the only person who was skilled in the art of “letterlocking” – the technique became common throughout Europe during the Late Middle Ages (1250-1500) and Early Modern periods (1500-1815). By folding and cutting letters in various clever patterns, people attempted to hide their correspondence from unwanted readers, and the “locks” came in myriad types.”This isn’t something special that people do on special occasions. This is how you send a letter before the envelope is invented,” explains Daniel Starza Smith, a lecturer in Early Modern English literature at King’s College London. “So, if it’s a business letter, if it’s a love letter, if it’s a spy letter, if it’s a diplomatic letter, they’re all using letterlocking. So it’s not something confined to experts, royalty or spy masters. Anyone who is capable of sending a letter is using letterlocking.”