Like the COOL Concept Piece Base Layer, the Craft Active Extreme Concept Piece Base Layer provides exceptional comfort when worn under a jersey due to its Moving Wing Technology, which stretches across the back to accommodate the specific range of movements a cyclist makes while riding. But the Active Extreme Concept Piece Base Layer is designed for cooler days, when moisture management is critical but a small amount of insulation can take the chilly edge off. The Active Extreme Concept Piece Base Layer is made of two fabrics. The shoulders and upper back are covered by Craft's lightest, most airy polyester mesh, with a portion of elastane that allows it to stretch. This fabric is also resistant to odor. The second fabric is 100 percent polyester with two layers: an inner layer with hexa channel fibers that wick moisture and increase airflow between the fabric and the skin, and an outer layer, which keeps moisture moving away from the body but also provides some insulation. The Craft Active Extreme Concept Piece Base Layer has an ergonomic, body hugging fit. It comes in sizes Small through XX Large, and in White.