Mummy bags generally have a ¾-zip design to retain more heat. If you sleep cold, you may feel very snug and warm with a ¾-zip bag. Most rectangular bags, however, have full-zip designs. The nice part about full zips is that you can completely unzip the bag and use it as a quilt—a nice option if you want to cover up the kids for a nap while you’re driving. You can also zip two rectangular bags together if you need lots of room or you don’t like sleeping alone. If you want to use your full-zip as a picnic blanket, look for designs that have tough outer fabrics that can stand up to rough, abrasive use and ones that clean-up easily if something spills on it.