In mountaineering, a bivouac, bivy, or bivy sack, is an extremely lightweight alternative to traditional tent systems. Very popular among climbers and minimalist campers, a bivy sack at its barest is a thin waterproof fabric shell designed to slip over a sleeping bag, providing an additional 5-10 degrees of insulation and forming an effective barrier against wind and rain. Nowadays there also exists the bivy shelter, a compromise between bivy sack and single-person tent. Using hoops or poles, a bivy shelter is usually supported along its length just enough to keep the fabric off of the occupant, and especially to provide some additional breathing room around the head (a traditionally bivy sack typically cinches all the way down to the user's face, leaving only a small hole to breathe or look through.) Many campers gladly accept the increased weight of a bivy shelter for the huge increase in comfort it affords. However, the traditional bivy sack certainly still holds its place among climbers and backpackers, and is frequently carried on long or dangerous hikes and climbs as a compact emergency shelter.