evo.com | Smartwool Long Underwear One Piece > The Smartwool Classic Thermal Base Layer One Piece is the ultimate cold-weather cheat code. Made with 100% Merino wool, it keeps you warm, comfortable, and ready for full days outside without the bulk. A slim fit, flatlock seams, and a genius drop seat design make it the one layer you'll never want to take off. Slim Fit 100% Merino Wool Side Seams Wrap to Front Flatlock Seam Construction Set-In Sleeves | Smartwool Classic Thermal Base Layer One Piece Men's 2027 - Small Black