In occult mystery schools (aka Secret Societies) members/initiates use handshakes, codes and symbols to communicate with one another and more importantly, to hide things in plain sight.
Here's a link to a post on this site from 2011 with 2 videos explaining The Numeric Language of Secret Societies.
Here's a link to a post on this site from 2011 with 2 videos explaining The Numeric Language of Secret Societies.