Mummy 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. Some bags are designed especially to accommodate women's body shapes. Most mummy bags do not unzip all the way to the feet, because the zipper is a weak point in any sleeping bag's insulating qualities. Together with the tapered shape, this design feature helps protect the feet, which are more vulnerable to heat loss than other parts of the body.[citation needed] Another design feature is a drawstring, equipped with a cord lock, at the head end to help prevent the escape of warm air. A mummy bag often cannot be rolled neatly like a rectangular bag. Instead, it is simply stuffed into a stuff sack or compression sack. The bottom of a sleeping bag typically does not provide significant insulation, because body weight crushes the loft of the insulation material. It is therefore necessary to use a pad or other less crush-able insulation underneath the sleeper, especially in cold weather. Due to this, some sleeping bags do not include insulation on the bottom. Some include a sleeve for holding a sleeping pad. Additionally, some campers, especially ultralight backpackers or hammock campers, have started to use a top quilt, essentially a sleeping bag without a back. Some top quilts include a foot box, while others are just simple blankets.