The RFL is the lightest sock we make for the hill but it's no lightweight. It's 100% Darn Tough. Coveted by performance skiers and riders for its minimalist construction, the RFL provides plenty of warmth and a smooth, streamlined fit. This sock just disappears when you put it on. Ideal for skiers and riders who are looking for a sock that interfaces seamlessly with their boots and allows them to feel the terrain underfoot. THERMOLITEÂ® fabric and insulation provides warmth and comfort without weight. Still made in Vermont. Guaranteed for life.