The Patagonia Capilene Layer system is a perfect system for varying conditions. The interchangeable thicknesses are all designed to be comfortably substituted based on any type of weather. The Capilene Thermal Zip Neck (this one) is the warmest of the layering options. It's perfect for wearing as a lightweight mid layer on warm days, or as a next-to-skin layer on those bitterly cold days. The tall zip neck collar protects against drafty wind and allows for temperature control. The fabric is a blend of polyester fleece and spandex, a combination that allows for unrestricted movement and complete comfort. [+]