This is a roomy and stand alone sleeping bag for 3 season use with a center zip. The bag has more internal volume than our standard mummy sleeping bags. It is a very good alternative if you feel a normal mummy sleeping bag is too cramped. Because of the extra volume it is also perfect as an outer bag in combination with a sleeping bag you already have - and you get a proper winter bag combination, similar to our Polar Spitsbergen Dual sleeping bag. The Pro Storen Fiber will quite easily fit a -10 to -15℃ fiber or down sleeping bag inside and gives you a good winter option down to -20 to -30℃.