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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 the Capilene 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.
The type known as "thermal underwear" is made from two-ply fabric of either a wool layer and an artificial fibre, only wool or – again mostly in the U.S. – two layers of only artificial fibres, which uses trapped body heat to insulate against cold air.
An adjustable two-piece design is credited to Canadian Frank Stanfield, a native of Truro, Nova Scotia, who patented his design on 7 December 1915. In 1898, Stanfield and his brother John had developed a product called Stanfield's Unshrinkable Underwear for Stanfield's, their garment manufacturing company.
Etymology of "long johns" 
The manufacturing foundations of long johns may lie in Derbyshire, England, at John Smedley's Lea Mills, located in Matlock. The company has a 225-year heritage and is said to have created the garment, reputedly named after the late-19th-century heavyweight boxer John L. Sullivan; the company still produces long johns.
History of long johns 
Long johns were first introduced into England in the 17th century, but they did not become popular as sleepwear until the 18th century. They were supposedly called long johns after a famous knife fighter who fought in thermal underwear. They were first used as loungewear but then later became popular as sleepwear. Children also wore long johns as sleepwear, usually made out of cotton and polyester.
In 2004, Michael Quinion, a British etymologist and writer, postulated that the "john" in the item of apparel may be a reference to Sullivan, who wore a similar-looking garment in the ring. This explanation, however, is uncertain and the word's origin is ultimately unknown.