A poncho liner is a military issued piece of field gear that is used to provide additional warmth to the standard poncho. The liner can also be used as a blanket, a ground mat, or a makeshift sleeping bag in an emergency situation during mild weather and can easily fit into any backpack or rucksack. US troops during the Vietnam War highly accepted poncho liners because they provided warmth during cool tropical night and gave comfort when used as an expedient sleeping bag.