A mid-layer is the layer of clothing between the base layer (closest to your body) and the insulation or shell layer (sometimes called outer layer). They can be sweaters, fleece jackets, long-sleeved tops, hoodies, shirts or thin down jackets depending on the conditions and activity. On a typical trek, your body temperature changes frequently, so it’s a good idea to wear multiple mid-layers as part of the layering system, especially in winter. That way you are able to regulate your body temperature as you go.