The original string vests were made of quite thick string and had, I believe, there origins in war time. After the war they were used by mountaineers to trap warm air and were quite effective. Survived a night out at 13,500ft in the Alps wearing one in a storm. When they got wet the fabric soaked up water and was not good. Later they became refined and the best one I ever wore was a Damart version in their excellent chlorofibre.