The F2019 Liner Crew Sock in Black from Fits. 

Nothing provides that extra layer of warmth when you need it most like the Liner sock. This ultra-thin sock is made of extra-fine Merino wool and won’t slip, slide, or bunch. All of the warmth without the bulk. A perfect mate to wear underneath your other FITS® for extra warmth.

• Dynamic Toe Cup contours to the shape of your foot.
• Heel Lock provides heel security that limits the chances of a blister when combined with a properly fit boot.
• Double-welted top so the sock always stays up, doesn’t bind or slip.
• Crew height rises just below the calf