This is the most interesting item I've seen all week. From the The New York Times:
To safeguard the most important royal correspondence against snoops and spies in the 16th century, writers employed a complicated means of security. They’d fold the letter, then cut a dangling strip, using that as an improvised thread to sew stitches that locked the letter and turned the flat writing paper into its own envelope. To get inside, a spy would have to snip the lock open, an act impossible to go undetected.Catherine de’ Medici used the method in 1570 — a time she governed France while her ill son, King Charles IX, sat on its throne. Queen Elizabeth did so in 1573 as the sovereign ruler of England and Ireland. And Mary Queen of Scots used it in 1587 just hours before her long effort to unite Britain ended in her beheading.
The image embedded at the top is a modern reproduction; the linked NYT article includes photos of several historic locked letters. And here is a video showing how the letterlocking was performed: