Letters in the 1600’s didn’t look like the letters that we know of today. Envelopes weren’t mass-produced until the 1830s, so the person who wrote the letter folded them in such a way that it became its own envelope, a process that has been deemed ‘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. This article delves into the intricacies of letterlocking and how intelligence units learned to ‘crack the code’ on them without giving away to anyone that the contents of the letter had been copied.
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