Merino Wool is the original performance fabric. It comes from sheep that deal with frosty mornings and hot afternoons all year, so it's naturally good at keeping you warm when it's cold and cool when it's not. The fibers are fine enough that it doesn't itch, it pulls sweat off your skin, and it doesn't hang onto odor, which is why a lot of us wear the same shirt two days in a row. We add a bit of synthetic so it dries faster and lasts longer. We make products with Merino because sheep figured this out long before we did.