A mummy sleeping bag tapers towards the feet and is therefore more fitted than a regular rectangular bag improving heat retention. The hood of a mummy sleeping bag also helps keep in the warmth. A mummy sleeping bag also weighs less than a rectangular bag.
Most mummy bags will be single but it is possible to buy left and right handed bags that can be zipped together to create a double bag. Our Microlite and Summit sleeping bags are good examples of this.
The packed size and weight of the bag should also be taken into consideration. This is especially important if you plan to fit your sleeping bag into a backpack or will be carrying the bag long distances.