The Black Diamond Project Heavy Flannel is a men's flannel shirt woven from heavyweight, brushed organic cotton twill. The boxy cut leaves room for layers underneath, so it slides over a base layer or midlayer without pulling across the shoulders. What Our Gearhead(R) Experts Are Saying:"The thick cotton holds off the chill on cold morning belays until the sun clears the wall. When the temperature drops further, I put a puffy over it, because a flannel adds warmth without replacing insulation."