Long johns," also known as long underwear or thermal underwear, are a type of undergarment traditionally worn to provide an extra layer of warmth in cold weather conditions. They are typically made from a stretchy, insulating fabric such as cotton, wool, or synthetic materials like polyester or polypropylene. Long johns are designed to fit snugly against the skin to trap body heat and provide insulation.

The design of long johns typically consists of a long-sleeved shirt or top and full-length pants, although variations such as short-sleeved tops and three-quarter length bottoms also exist. The snug fit allows them to be worn comfortably under regular clothing without adding bulk.

Long johns are commonly used during outdoor activities in cold weather, such as skiing, snowboarding, hiking, camping, or any other winter sports. They are also worn by individuals who work outdoors in cold climates, such as construction workers, farmers, or military personnel.

In addition to providing warmth, long johns also offer moisture-wicking properties, helping to keep the skin dry by drawing moisture away from the body. This is particularly useful during physical activities when sweating can occur.

Overall, long johns are a practical and essential piece of clothing for anyone facing cold weather conditions, providing an extra layer of insulation and comfort to help keep the body warm and protected from the elements.