I am a newby on the forum and looked through the threadst for nonelectronic comms and did not notice a topic on that. I am a fan of the old western movies and always loved the "Indian" use of owl hoots, bird calls, wolf howls and so on for signals.. The old Bluejackets Manual had whole sections on flag meanings and semaphore signals. We tend to forget that while the battle of New Orleans was fought weeks after the peace treaty was signed in London, word had not yet arrived to either side, Napoleon on the other hand, with his semaphore stations, could get word from Russia to Paris in a couple of hours. System was not improved upon until the telegraph was put in. For the invasion of Normandy each soldier was given a small "crickit clicker", one click was challenge, two clicks was ok answer. They had many different units, from different countries speaking different languages, and it worked out fine. Jerry Young has used challenge coins and code words in several of his stories and they seem well thought out . Even if you have radio comms, how do you establish a contact with a new group and how do you know that an old group has not been compromised and is now a threat? Hand signals at the squad level or lower allow sound security and signals can be used to identify you to a sentry without revealing his location and a given light, flag, window shade, or whatever might indicate to a unit the security condition of a retreat or neighbor's site, before you are commited. While all of this is covered in small unit tactics by the military,they have a very different focus and establishing contact is not a question. .