Rectangle-shaped sleeping bags go as much back as the late 1800's when a mail order catalog pioneer Pryce Pryce-Jones came up with the suggestion to attach 2 rectangular carpets together with ties. He is taken into consideration to be the innovator of today's contemporary rectangular sleeping bag design. After the war he had several of these bags still left as well as made a decision to market them to the public.
Temperature rankings are quite crucial items of details to consider when picking a bag. And although mummy and also rectangle-shaped resting bags can cover a wide range of temperature levels, mummy bags are the common designs for the extreme polar temps such as those you would locate in the Andes.
Semi-Rectangular: I call this the cross. This backpacking bag is rectangle-shaped ahead and afterwards slenders along towards the feet. With this type, you obtain added space, but not as much weight as a true rectangular resting bag. For those who are claustrophobic, like me; we can reach sleep a little better inside a bag with added room in it and still have some benefit from the form.