The USMC intermediate sleeping bag was designed to be uséd with or without the 3-season bag and bivy to create a sleeping bag system for the U.S. Marine Corp. When uséd alone, the intermediate sleeping bag is rated to 15 degrees but may be uséd down to -20 when uséd inside the system making this a versatile bag for 3 and 4 seasons. The intermediate bag was also made larger in order to comfortably accommodate the inner bag making this a great bag when extra room is needed. As with all military sleeping bags, they are rated when the soldier is wearing a medium weight base layer. Building a good sleep system will allow you the options to always be ready. So, get out and enjoy the outdoors.