What is the difference between a watch cap and a beanie?

A watch cap, sometimes referred to as a ski cap, is a snug-fitted knit hat with a roll at the bottom. The term originates from naval Watchmen who wore these types of hats while on "watch" during cold weather. A beanie is very similar to a watch cap, although it does not have the roll at the bottom and fits closely on the head without additional material extending from the top.