The Sea to Summit Expander Liner is a roomy rectangular shaped sleeping bag liner with a built in pillow slip, constructed of a quality stretch knit polycotton, which makes it even stretchier and warmer than an ordinary cotton weave liner. The stretchy fabric of the Expander Liner allows it to move with you while you sleep, greatly boosting comfort and reducing tangles. The anti-microbial treatment keeps bacteria at bay while also extending the life of your sleeping bag and liner. Expander Liner - Traveller (with Pillow slip) - Green