The Patagonia Mountain Thermal Crew Top moves its shoulder seams off the strap line and adds full underarm gussets, so a loaded pack doesn't turn into a source of chafing on a long day. Polartec Power Grid fabric backs that comfort with an open-grid structure that wicks moisture and holds warmth without trapping heat, built to layer easily over a base layer underneath. What Our Gearhead(R) Experts Are Saying:"I've worn this under a loaded pack for six hours straight and never once reached back to adjust a strap that was rubbing. The underarm gussets let me reach for a trekking pole without the fabric pulling across my chest."