Yes, you know Smartwool socks, but do you know Smartwool's base layers? This version of Smartwool's key layer top is done a very fine Merino wool blend, providing lightweight warmth when there's a nip in the air. The top has long sleeves and a flatlock seam construction that helps prevent chafing; the fit is quite close. Merino wool naturally helps regulate body temperature, transports sweat away and cancels out odors. We find that this runs a bit on the small side; many of our in-store customers opt to size up in this style.