Base layers are part of the layered clothing system which is used to find the most appropriate combinations of layers of clothing to suit the climatic and environmental conditions.
Layered clothing is a term describing a way of dressing using many garments that are worn on top of each other. Some of the layers have different, largely non-overlapping, functions. Using more or fewer layers, or replacing one layer but not others, allows for flexible clothing to match the needs of each situation. Two thin layers can be warmer yet lighter than one thick layer, because the air trapped between layers serves as thermal insulation. Layered clothing is particularly relevant in cold climates, where clothing must at the same time transfer moisture, provide warmth, and protect from wind and rain. In a hot and dry climate, clothes have very different functional requirements: they must block the radiation from the Sun, and allow for sufficient air circulation. Therefore, layered clothing in the sense used in this article is largely irrelevant to hot and dry climates.
Outdoor and sports wear manufacturers favor layered clothing because, among other reasons, it allows them to offer so-called “technical” or “functional” clothes which are optimized for the particular demands of a specific layer. Such clothes are often made of advanced synthetic materials, and can be expensive.
Usually at least three layers are identified as follows:
Inner layer provides comfort by keeping the skin dry. Also called base layers or first layers.
Mid layer provides warmth. Also called insulating layer.
Shell layer protects from wind and/or water. Also called outer layer which works as protection over the other two layers.
Often clothes combine two adjacent layers, as in the case of warm undergarments that provide both comfort and insulation.