Unusual Backpack

The most common materials for such packs are canvas and nylon,(polyester), either ripstop fabric for lightweight packs or heavier fabric such as cordura for more typical usage. Most such packs are purpose-designed for the outdoors market; however, it is not uncommon for military surplus packing gear to be sold to outdoorspeople as well for the same purpose. The cheaper versions of the outdoor packs are often favoured by city trekkers; as they have a large volume and still carry relatively easily.
Outdoors packs, in addition to the distinction between external-frame and internal-frame, can be further subdivided based on the duration of trip a pack might be expected to be used on; daypacks hold supplies for a single day's hiking (size about 20-30 litres), while "weekender" bags can hold two to three day's worth of gear and supplies (sizes about 40-50 litres). Larger packs generally have no specific names but are designed to distribute the weight of increased numbers of gear and supplies for longer-duration trips (60-100 litres); such packs often include complex ergonomic support features to simplify the carrying of large amounts of weight. A third type with little or no frame at all, similar to the bookbags used by students and made of light fabric (often nylon ripstop, as mentioned above), is used in ultralight backpacking to eliminate the weight of the frame and heavy fabric used in more typical outdoors packs. Despite (or perhaps because of) their lesser weight, such packs are seldom less expensive than more typical, regular-weight packs.
In addition, outdoors packs are designed for specific purposes such as kayaking/canoeing, rock climbing, mountaineering, cross country skiing, and other such activities. Hydration packs are small backpacks containing a bladder-like plastic sac to hold up to three litres of water or any other beverage for hands-free drinking; they are sometimes known as camelbacks, from the CamelBak company, the best-known manufacturer of the packs. Packs used in competitive strategic sports such as paintball and airsoft are often based on or actually are military gear.