A fallout shelter is an enclosed space specially designed to protect occupants from radioactive debris or fallout resulting from a nuclear explosion. Many such shelters were constructed as civil defense measures during the Cold War. These signs were posted on buildings with shelters. All Department Of Defense and Offices of Civil Defense issued signs have 'Not To Be Reproduced Or Used Without Department Of Defense Permission.' at the bottom of the sign. They generally measure 10 by 14 inches and are made of aluminim or galvanized steel. There were several variations.