Long underwear, also called long johns, or thermal underwear, is a style of two-piece underwear with long legs and long sleeves that is normally worn during cold weather. It offers an advantage over the one-piece union suit in that the wearer can choose to wear either the top, bottom or both parts depending on the weather. Long underwear is also less commonly known as long handles. It is commonly worn by people in cold countries.
Modern long underwear has largely supplanted the union suit. In the United States, it is usually made from a cotton or cotton-polyester-blend fabric with a box-weave texture, although some varieties are also made from flannel, particularly the union suit, while many newer varieties are made from polyester, such as theCapilene trade name.European manufacturers use wool blends or even 100% wool, usually Merino or other high-quality wool. Some models might include a thin layer of polyester to transport moisture away from the skin. Wool, in addition to being fire retardant, provides highly effective insulation and will keep its insulating properties even when wet, as opposed to cotton.