Rectangular, square or envelope sleeping bags as they are also known, are the traditonal shape of bag which many people still prefer. Although mummy shapes offer better insulation, rectangular bags feel less restrictive and generally don’t have hoods. They are a popular choice for use in caravans and sleepovers.This type of sleeping bag is the same width at the top (head end) as it is at the bottom (foot end), is usually zipped down the entire length of one or both sides and has squared off, right angled corners.Separate bags can often be zipped together or laid out flat and used as an over- or under-blanket. As there is lots of spare space in rectangular bags they are the least efficient shape for keeping you warm and are typically used as summer sleeping bags in low altitudes. As they use lots of material they are often, although not always, the heavier and bulkier type of bag although the sleeping bag’s filling will play a significant part in this too. Some people who are less susceptible to the cold and prefer to be able to move about freely inside their sleeping bag may opt for a rectangular bag.