thumb|A locked letter from 1603
thumb|A locked letter from 1603
Letterlocking is the act of folding and securing a written message (such as a letter) on papyrus, parchment, or paper, without requiring it to be contained in an envelope or packet. It is a traditional method of document security that utilizes folding and cutting. The process dates to the 13th century in Western history, corresponding with the availability of flexible writing paper.
Discovered by embedding cosine similarity (sentence-transformers MiniLM, 384-dim).