Letters folded into their own security locks

Before envelopes, people could turn a letter into its own lock. Folds, slits and paper strips secured the message and could show if it had been opened. Some designs threaded a strip cut from the letter through the folded packet. Researchers read a sealed letter from 1697 without opening it: X-ray scans captured its folds, and software unfolded the sheet virtually. The message survived, along with the original paper lock.

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