Letterlocking

Letterlocking is the practice of folding, slitting and sealing a written sheet so that it becomes its own envelope, and so that opening it leaves visible damage. It was ordinary correspondence security from roughly the thirteenth to the nineteenth century, before gummed envelopes separated the message from its container. Systematic study of it is recent, led by the conservator Jana Dambrogio with the historian Daniel Starza Smith.

That scholarship supplies vocabulary the practice never had, and a working Taxonomy of it. A format is the finished geometry; a technique is the operation — folding, tucking, slitting, sewing, sealing with wax or with a strip cut from the letter itself. Formats include the tuck and seal, the pleat, and the spiral lock, in which a sliver sliced from the sheet is threaded through stab holes and sealed, so the sliver must be torn to open. A dagger trap is a booby-trapped variant whose slits are cut so that any route in but the intended one rips the writing surface.

None of this prevents interception; all of it makes interception legible. That is tamper evidence rather than secrecy — the difference between a Hash Function and encryption, or between Code Signing and access control — and a seal that can be lifted and replaced invisibly is a Silent Failure of the entire scheme.

Modern work reads locked letters without destroying them: X-ray microtomography and a computational unfolding algorithm recovered text and fold sequence together from sealed, undelivered letters in a seventeenth-century postmaster's trunk. The creases are themselves evidence, so the fold carries Provenance alongside the words, and a proposed reconstruction can be tested by refolding — unusual Falsifiability for a historical claim. Opening the letter destroys a Ground Truth no transcript restores.