Synergy is Smartwool's technical midlayer fabric. The 60% SmartWool/40% nylon double knit gives you wool on the inside and nylon on the face. This allows for extra warmth with maximum breatheability and stretch. A desired blend for aerobic activities in winter, the flat face of the fabric slides easily under other layers. This pullover style has a zip collar for venting, no shoulder seams and a zippered chest security pocket.