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Tapered: Not as shaped as a mummy bag. The zip may go to the bottom, and some models may have another zip across the bottom, to let them open out as a quilt. Alternately, you can zip two similar bags together (but this rarely works very well in practice).
Mummy: Shaped like an Egyptian mummy, with a zip down the side. The shorter the zip, the better: less weight and less heat loss. Usually designed for the coldest conditions, snow and ice, and with a good hood and neck muff. Australian mummy designs have more sideways leg room than American ones, which seem incredibly narrow. Maybe Yanks have no leg muscles?
Tapered: Not as shaped as a mummy bag. The zip may go to the bottom, and some models may have another zip across the bottom, to let them open out as a quilt. Alternately, you can zip two similar bags together (but this rarely works very well in practice). The bottom zip is excess weight in my opinion. At least one manufacturer agrees with this comment, but says the market still asks for it. Still designed for fairly cold conditions, and with a fairly good hood.
Rectangular: This is designed to open out into a quilt, with zips down the side and across the bottom. The design is unreliable in the cold, but these bags are not meant for really cold conditions anyhow. Most have a hood, but not all. Suitable for warm conditions (Northern Territory in summer) and travel.
Single sided: This is a variation on the rectangular design. In the interests of saving weight only the top half of the bag has baffles and down in it. The bottom of the bag is either a sleeve into which you insert your sleeping mat or just a single layer, like a sheet. There is some weight reduction in this design, but not everyone finds the result adequate. However, the reasons people have trouble with this design are varied. If you try to treat it like an ordinary bag, you may have some problems. In addition, with the weights you find with some older shells, leaving just the down out is purely a cost-cutting exercise: it doesn't save much weight. Skip forward to the Quilt design below.
Double width bags: Sounds a wonderful idea (hint, hint), doesn't it? But in practice, there are complications, especially in the snow with conventional designs. For a start, in general you cannot get the separate hoods to do up around your heads very well. There are drafts in the gap between the two of you. If one person sits up in the night for any reasons, the other person gets cold. A double-width liner is a disaster: the liner tends to stick to your sweaty skin, and after one person turns over you are both tied up in knots in the liner. Still, have fun trying. On the other hand, if you have separate liners a double bag may still work if it is not very cold. On the other hand, a double-width quilt is just the same as a doona at home.
Lightweight bags: This doesn't refer to cheap supermaket bags but to specialised bags made for experienced walkers, usually with very high quality materials. After all, you don't always have to carry a -30C snow bag with 900 gms of down. They can come in a variety of shapes, and are sometimes sold as liner bags for really extreme conditions. We have used a Mont Nitro bag with only 150 gms of down at temperatures down close to freezing. When it got very cold we wore thermal tops inside these bags, and my wife and I slept snuggled together for extra warmth.
More recently I made my own shells out of Pertex Microlight fabric. The shells weigh about 250 grams: a far cry from the 850 grams you find in some Australia bags of the older vintage. And I put 300 grams of 800 loft down in the shells: better than the cheaper stuff found in many Australian bags. My wife and I have used these bags across the Pyrenees in summer, in NSW in the thrtee warmer seasons, and even in Kosciusko NP when it went sub-zero over night. We were OK with thermals and snuggled up. You can get imported bags like this, and One Planet have started (mid-2006) making some commercially - the Cocoon series. Don't try going this lightweight unless you know what you are doing.
Quilts: I distinguish between single-sided bags and quilts for good reason. A quilt is really meant to function like a doona on your bed, not as a bag. Your mat, be it foam or inflated, provides the insulation under you and the quilt goes over you. This puts the insulation where you need it - and dispenses with the weight of zips as well. ""What about a hood?"" you might ask. Well, the bit of the hood between your head and your pillow does nothing. It's squashed flat, and wasted. What do you do at home? You pull the doona over your head. What did people do in the old days before central heating? They wore a bed cap. What do I do? Both, in fact. I have a fleece hat I wear in bed, and if it's cold I pull the 'hood' part of my sleeping bag over my head. This is because I use my Pertex-shell sleeping bag as a quilt, on top of me. This works down to sub-zero conditions with 300 grams of down.
Now to get very clever. For very cold weather - like seriously sub-zero in the snow, we now take our light Pertex-shell bags plus a 930 g quilt (shell 330 g, down 600 g). My wife and I sleep snuggled up with our individual light Pertex bags with the quilt over the two of us. There is enough insulation from the two layers to keep us warm, but in addition I have a heater with me under the quilt: my wife. The total mass of the two bags and the quilt is 2.05 kg, while the combined weight of our two old conventional snow bags was 3.36 kg - and we are warmer.