A short person or a child might be able to get by with a normal sized sleeping bag by tying off the lower portion with twine or camping straps. The rectangular ones have more room to move around in. Generally, they are the least expensive and easiest to zip together to make a double bag for two people, but they are also the bulkiest and heaviest. Mummy bags tend to be warmer because there is less space to heat. They also take up less room in a stuff bag and are the most heat-efficient.
Tapered bags fall between the two.
They can be zipped to others with matching zippers, take less body heat to warm them than rectangular ones, and allow some room for foot movement. Most bags are rated according to the lowest temperatures in which they provide comfort, but there is no set standard in the industry. The rating is determined by the manufacturer. Goose down is a natural fiber that has long been held as the standard as lightweight, comfortable, and warm. They compact easily into small stuff bags, mold easily around a camper's body to retain heat, and are extremely durable.