The Chinook The Beast -40F is a heavy duty multi-season sleeping bag due to its two detachable layers. You basically get 3 in 1: The first is the outer, heavy duty cotton canvas layer filled with 3.4 lbs. of Insufil 4 Thermofiber and cosy poly-cotton. This serves as excellent mid-range all weather sleeping bag. The inner, detachable flannel liner makes for an excellent hot-weather sleeping bag. And both together keep you nicely toasty when it gets really frosty at night. The rectangular shape gives you lots of room to move around and the extra-large zippers let you in and out easily. It's the perfect rugged sleeping bag for multi-day fishing trips, car camping or to keep at the hunting lodge.