Long before email, text, and instant message, important words were passed discreetly from closed palm to palm. To keep the messages private, these hand-written notes were often elaborately folded, sealed with wax, and rigged with anti-tamper devices. The technique of “locking” letters involves folding the parchment, papyrus, or paper securely so that the letter functions as its own envelope.
To seal a modern-day envelope (on the off chance you’re sealing an envelope at all), it takes a lick or two, at most. Not so prior to 1891 when F.W. Leslie patented the envelope seal. Or most of history, letters were folded in such a way that they served as their own envelope. Depending on your desired level of security, you might opt for the simple, triangular fold and tuck; if you were particularly ambitious, you might attempt the dagger-trap, a heavily booby-trapped technique disguised as another, less secure, type of lock.
These locks placed on you letter didn’t prevent others from opening your mail but if they did, you had a way of knowing it had happened. To a spy, or a spymaster, this is critical knowledge. Using small slits, careful folds and tabs, and even string or ribbon, the author of a letter that is “letter-locked” is able to devise boobytraps that must be broken before the letter is read. If the recipient sees these protections foiled, they know the letter has been read by someone before it was received.