Mammut Arctic Midlayer Jacket - Men's: Over or under. The insulating Arctic ML Jacket is the perfect cozy fleece to wear under a hard shell jacket and can also be worn as a breathable outer layer. The integrated PolartecA(R) Thermal ProA(R) material has large pores and stores air that has been warmed by the body, which means it offers excellent heat retention. The material also dries extremely quickly after demanding ascents. The ideal heat/weight ratio, flatlock seams for extra elasticity and pre-shaped elbows make the Arctic ML Jacket a fleece for sporty winter pursuits. Features High-quality PolartecA(R) Thermal ProA(R) fleece offers ideal warmth-to-weight ratio Flatlock seams for reduced bulk and extra elasticity Concealed flat chin protector 2 side pockets with zipper