The Sitka Merino Lightweight Bottom features premium 17.5-micron merino wool spun around a nylon core, which produces a high-performance next-to-skin layer that combines the comfort, warmth, moisture management and natural odor control of merino with the durability and elasticity of nylon. With a high heat-to-weight ratio and excellent breathability, the Merino Lightweight Bottom excels as a versatile base layer to provide just the right amount of insulation.