From Pattern Notes:

If you live in a colder climate, mittens are a simple fact of life. You need to keep your hands covered for at least a couple of months of the year. Even as the weather warms and people begin to shed their winter jackets, mittens can be useful in keeping the wind at bay. Hence the Simple Mittens pattern. Good old mittens. It’s true that mittens are warmer than gloves, but they do rather hamper dexterity. So I’ve whipped up some alternative features to meet special needs. The Bad Habit mitten. These used to be known as hunter’s mittens, with a separate index finger to operate the trigger of a hunting rifle. They’re just as useful for that equally politically incorrect smoking habit. And flip-top convertible mittens, designed for all the mothers I know. The top flips open to expose the fingers, making it easier to buckle up the car seat on cold winter mornings.