A sleeping bag is a protective "bag" for a person to sleep in, essentially a blanket that can be closed with a zipper and functions as a bed when camping, hiking, or climbing. Its primary purpose is to provide warmth and thermal insulation. Sleeping bags also protect, to some extent, against wind chill, precipitation, and exposure to view. The bottom surface of a sleeping bag provides some cushioning, but a sleeping pad is usually used in addition for that purpose.
A basic sleeping bag is simply a square blanket, fitted with a zipper, allowing it to be folded in half and secured in this position. The basic design works well for most camping needs but is inadequate under more demanding circumstances. The second major type of sleeping bag, sometimes called a mummy bag because of its shape is different in a number of important ways. It tapers from the head end to the foot end, reducing its volume and surface area, and improving its overall heat retention properties.
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