I believe the onion-skin paper had two advantages for messages intended for courier transport: it was really light, so more messages could be carried per courier (this became even more important for air-mailed communications; more messages per plane!); and it was also, for high-security messages, more easily folded up and concealed. (You might guess it was more easily destroyed under threat, too, but it&#x27;s probably not—onion-skin paper is stronger than it looks!)<p>Oddly enough, in the military-fiction book I have on my lap at the moment, one of the characters has a rolled-up bit of onion-skin paper tucked into a hidden compartment of their jacket with a set of recent codes and ciphers on it. Similar idea.