The 300 MerinoFine Polar Long-Sleeve Roll Neck Top uses icebreaker's finest 15. 5-micron merino wool at their heaviest 300 warmth rating, built specifically for the coldest days out rather than shoulder-season layering. A high roll neck adds coverage at the collar, while thumb loops anchor the sleeves over the hands to seal out cold air at the wrists. Raglan sleeves and flatlock seams keep the fit moving cleanly under a shell or insulated jacket. What Our Gearhead(R) Experts Are Saying:"I wore this under my shell on a below-freezing lift day and never touched my neck gaiter once, the roll neck came up high enough on its own. My hands stayed warm too since the thumb loops kept the sleeves from riding up under my gloves."