Available in various sizes to suit most body shapes, this style of sleeping bag has a square bottom with the same width top and bottom allowing the most comfort.
The more technical come with a hood and mostly the comfort and patterned bags come without. Often two of the same rectangular sleeping bags can be zipped together to offer one oversized double.
If you move a lot in your sleep, or spread out then a rectangular sleeping bag is normally the best allowing you the extra room for movement.