Long underwear, often called long johns, is a style of two-piece underwear with long sleeves and long legs that is normally worn during cold weather. It offers an advantage over the union suit in that the wearer can choose to wear either the top, bottom, or both parts depending on the weather. Long underwear are also less commonly known as "long handles".
Modern long underwear has largely supplanted the one-piece union suit. 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 polypropylene, such as the Capilene trade name. The type known as "thermal underwear" is made from two-ply fabric, which uses trapped body heat to insulate against cold air.
Sir Benjamin Thompson, Count Rumford has been credited with inventing thermal underwear.
An adjustable two-piece design is credited to Truro, Nova Scotia native Frank Stanfield of Stanfield's Limited who patented his design on 7 December 1915. Earlier, in 1898 Frank Stanfield and his brother John had developed a product called Stanfield's Unshrinkable Undewear.