The multi-layer system involves layering several pieces of clothing to combine their effectiveness and thus maintain optimal body temperature during outdoor activities. There are usually at least three layers: a base layer that absorbs moisture and wicks it away, a mid layer that retains heat and lets moisture circulate outwards, and an outer layer that protects from the wind and precipitation while also letting moisture out. Layers of clothing can be removed or added as needed, depending on the weather and the course of each activity.