The Icebreaker 300 MerinoFine Polar Legging is a heavyweight merino base layer bottom built for the coldest days outside. Its 15. 5-micron MerinoFine fiber is fine enough to sit soft against skin while still delivering the warmth of a heavyweight weave, so the legging holds heat without feeling stiff or bulky under a shell. What Our Gearhead(R) Experts Are Saying:"I layer these under a shell on the coldest mornings and forget I have them on. The gusset gives my stride full range without binding, and the flatlock seams never show up as a hot spot after hours on my feet."