Long John probably the most important layering needed to fight the blistering cold. A style which consist of two-piece underwear with long legs and long sleeves that is normally worn during cold weather. It offers an advantage over the union suit. The individual wearing it can choose weather they want to wear the lower or upper or both depending on the weather.
Modern long underwear has largely supplanted the one-piece union suit. In the USA, 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.