It takes a Swedish company to make base layers that are only supposed to come out when the temperature hits the 30's (and lower!) That's the beauty of the Craft Women's Pro Warm Mock Neck Base Layer. It's rated for use from 30 degrees down, and it's an essential piece of your wardrobe when you choose the great, icy outdoors over another torture session on the windtrainer. Pro Warm is made from what Craft calls a "3D knit thermal yarn." A quick glance at the Pro Warm makes it obvious that different panels are doing different jobs, and that's exactly how Craft designed it. Anatomically mapped zones serve specific purposes -- some trap warmth, others vent to wick moisture.Take a good look at the image of the Pro Warm top. The lighter colored sectors are the better-insulating areas. The darker-colored sectors are the places where the fabric is a lighter weight and greater moisture transpiration occurs. The "mock" of the mock neck means it's a tall neck without the extra material to fold over ala a "turtleneck." The actual Base Layer has a 3/4 length zipper. Although this is not shown in our image, it is how you will receive the Pro Warm Zip Mock Neck Base Layer.