Western Mountaineering's Bristlecone MF Sleeping Bag is a semi rectangular bag built for subzero temperatures. Ideal for most winter endeavors, the Bristlecone's 850 down insulation has the loftiest fill rating, resulting in the highest warmth to weight ratio available on the market. Unlike most water resistant shells, Western Mountaineering's MicroLite XP fabric offers long lasting weather protection. The MicroLite XP shell helps the down retain its lofty warmth in moisture riddled environments. Western Mountaineering critically thought out the bag's design to the smallest details, making the Bristlecone a long lasting and very versatile choice. A differential cut places more fabric at the lining to ensure maximum loft inside the bag, and continuous baffles allow you to shift the bag's down towards the top or bottom for more or less warmth. Hook and loop tabs let you pull the hood, draft tube, and draft collar farther or closer to you for customized coverage. Extra convenient features include dual zippers on the left or right side, a waterproof stuff sack, and a storage sack with a large volume that helps the bag retain its loft over time.