life preserver, a personal flotation device (PFD) intended to keep the wearer afloat, particularly in case of shipwreck. A Type I PFD will keep even unconscious people afloat in a face-up position; it is the most common type used at sea. Another common type, developed during World War II for fliers and called the Mae West (named for the actress because of its shape), is made of inflatable rubber; it is still carried on commercial aircraft. Other types of life preservers are meant to be used only as a stopgap by a conscious wearer; these take the form of rings, cushions, or vests, and are either inflatable or filled with buoyant material such as unicellular foam, fibrous glass, or kapok. The large balsa wood life rafts once carried by all ships have been replaced by canisters containing inflatable life rafts capable of holding from four to twenty-four people. Most countries require that ships, and airplanes crossing the water, must carry life preservers and that crew and passengers must be drilled in their use.