The Basis PeakWool Mid 1/2-Zip Top is a merino wool base layer that blends RWS-certified merino, TENCEL, recycled nylon, and spandex into one midweight fabric that holds its shape and warmth over repeated wear. A high-efficiency grid backer traps warm air against the skin and pulls moisture through to a quick-drying outer face, and the half-zip neck vents heat the moment effort picks up. Flatlock seams and thumbholes round out a layer that disappears under a shell without added bulk. What Our Gearhead(R) Experts Are Saying:"My old 100% merino tops pill out at the pack straps by January. This one's still smooth there after a full winter of back-to-back tours, and it's just as warm as it was on day one."