The ultimate thermal essential, Smartwool® Classic Thermal Merino Base Layer One-Piece. Crafted with slim-fit contours that follow your body's natural shape, this one-piece features a mock neck and long sleeves for full coverage without bulk. The interlock-knit merino fabric excels at moisture management, temperature regulation, and odor resistance, ensuring all-day freshness. Thoughtful design touches like flatlock seam construction minimize chafing, while set-in sleeves and side seams wrapped to the front provide a distraction-free fit for every adventure. Need convenience on the go? The zippered drop seat with a fully functional fly adds practical functionality without sacrificing comfort.