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Below 0 degrees
Select a sleeping bag with a lower temperature rating than the lowest nighttime temperature you expect on a trip. On warmer nights, you can always unzip it to cool off a bit. Note that even though down bags are considered warmer in general -many factors go into bag warmth, so the temperature rating is your best indicator of warmth.
Synthetic insulation is found in more affordable bags and can be a good choice because it continues to insulate when damp.
Down insulation is more expensive but performs exceptionally well in cold, dry weather. For cold damp conditions, you can look for down that's treated to be water repellent. Because down packs small and is warmer for it's weight than synthetics, a lot of backpacking bags use it.
Camping bags offer plenty of room to move around in, while backpacking bags are light and snug. If you'll be using one bag for both activities, choose a backpacking style because you need it to be lightweight if you're going to carry it in a pack. Read How to Choose Sleeping Bags for Backpacking to learn more about bags for the backcountry.
Although some bag makers are experimenting with alternative shapes, most sleeping bags come in one of the three following shapes:
Rectangular: These bags allow plenty of room for both legs and arms to stretch out; some can be completely unzipped and used as a comforter.
Semi-rectangular: Also known as a modified mummy or barrel shape, this designation covers a variety of shapes, all of which offer a compromise between warmth and roominess.
Mummy: In order to boost warmth and cut weight, this bag style has a snug fit so you roll over with your bag rather than inside of it.
Double sleeping bags are the best bet for couples who plan on sleeping together-they need to be the same model and brand though! A few sleeping bags also can be zipped together if one person chooses right-hand zip while the other chooses left-hand zip.
Kid-size sleeping bags are typically shorter, smaller variations of adult-sized sleeping bags, though they might also have features like zip-open bottoms that let a child walk around in their bag, or provide a way to cinch up the bottom of the bag to be cozier for a smaller child.
A hood is typically found on bags with lower temperature ratings to make them warmer; most hoods have drawcords that let you snug them tight for more warmth. Bag zippers help adjust ventilation; some nicer bags prevent snagging by having a guard along the length of the zipper. Stash pockets keep personal items like watches and lip balm close at hand.