Mass-produced envelopes weren’t invented until the 1830s, meaning that most 17th-century letter writers folded their correspondence in such a way that it became its own envelope—a process Dambrogio had dubbed “letterlocking.” Letterlocks could be simple, just a series of quick folds without any sort of adhesive. But they could also be incredibly complex, even booby-trapped to reveal evidence of tampering. ~ From Before Envelopes, People Protected Messages With Letterlocking ~ Atlas Obscura
You just *know* I’m going to be practicing letterlocking now. And I plan to introduce it in “The Hidden Doors”, Book 3 of the Brassbright Chronicles. I can’t do it in Book 2 because I *really need* envelopes to be a thing for the moment. But, I have devised a way to have my world start reviving the old craft of letterlocking as the stories move forward.