Add versatility to your Western Mountaineering sleeping bag with their Three Season Bag Expander. Designed to zip into any mummy-style Western Mountaineering sleeping bag, the expander gives you a little more elbow room. Western Mountaineering has stuffed this bag expander with the same 850-fill-power goose down found in their sleeping bags. The expander works best with 6 foot bags, but will work with any length, with a bit of overlap (most of the length difference between the bags is below the zipper in the feet).