The Western Mountaineering Sleeping Bag Expander can be added to your favorite Western Mountaineering sleeping bag to create a bit of extra elbow room, or breathing room during warmer weather camping trips. The bag expander is also great for providing just the right amount of space to squeeze into a sleeping bag with that special someone for some world class snuggling. This sleeping bag expander is technically compatible with Western Mountaineering sleeping bags only, so if you buy it thinking it will work with another brand's sleeping bag and it doesn't, it's your own fault.