The North Face Baselayer system is designed to fit close to the skin and efficiently transfer moisture to your secondary or outer layers where it then evaporates. The ultra-lightweight hollow core Polypropylene yarns have a Microfibre yarn wrapped around them that acts as a pump to increase movement of moisture from the body. This also traps body heat. Together, this creates a substantial feeling garment with no increase in weight. The Elastomer yarns add stretch, giving a close, comfortable fit that wears well and maintains its shape through use.