A backpack (also called rucksack, knapsack, packsack, pack, Haversack, or Bergen) is a bag put on somebody's back.  It usually has two straps that go over the shoulders.  It is used to carry things in it, and it often has many pockets or compartments to carry things.  People often use backpacks when camping, hiking, or other activity where they need to carry many things.  Backpacks are also be used in the military by soldiers.  When used in school the are called bookbags or school bags.

Large backpacks, used to carry loads over 10 kg (22 lbs), and smaller sports backpacks, usually offload the biggest part of their weight onto padded hip belts. This leaves the shoulder straps mainly for keeping the load in place. This makes it easier to carry heavy loads, because the hips are stronger than the shoulders. It also improves agility and balance, because the load lies closer to the center of mass of the person wearing it.

In the distant past, backpacks were used to carry hunters' larger catches. If the hunts were even larger, the hunters would butcher their prey into pieces and hand out the pieces to other hunters.  The hunters would then carry smaller pieces separately.  The bag would be made up of animal skin and sewn together by animal intestines. They would then be woven together tightly to make a firm material.

Some backpacks are specialized to carry a particular thing, such as fluids or a laptop computer.

Designs 
Most backpacks can be closed with a buckle, a zipper, or a dry-bag type closure.  

A backpack may or may not have a pack frame.  If it does not, it is frameless. If it has a frame, it may be external or internal.  A pack frame supports the pack and scatters its weight across the body.  Most of the weight is taken off the shoulders.  This way, users are less often injured, and the upper body moves freely.

Frameless 
The simplest backpack design is a bag fixed to two shoulder straps.  The simplest designs consist of one main pocket.  This can be combined with webbing or cordage straps.  More complicated models add extra pockets, straps, and padding.  Sometimes they have reflective materials for safety at night.  These packs are usually cheap to make.  They are not suited for heavy loads.

External frame packs 
External frame packs were designed to carry heavy loads (>20 kg or 40 lb). This gives the wearer more support and protection and distributes weight better. Wooden pack frames were used for a long time around the world. Ötzi the Iceman may have used one in Copper Age Alpine Italy. But some archaeologists think the frame found with the body was part of a snowshoe. These packs are common in military and mountaineering. Metal packs first appeared in the mid-20th century.

The frame of an external frame pack is made from aluminum or other light material. It also has straps and a stretched netting which keeps the metal frame off the user's back. External frame packs have a fabric "sack" part which is usually smaller than the sack of internal frame packs. Nevertheless, they have exposed frame portions above and below the sack to make it easier to hold larger items. In addition, the sack can often be removed entirely. This lets the user change the arrangement of his load, or to move an unusual load. Military packs are often external frame designs because they can carry loads of different shapes, sizes and weights.

Internal frame packs 
The internal frame backpack is a recent creation, invented in 1967 by Greg Lowe. An internal-frame pack has a large fabric part around an internal frame. The frame is made with strips of aluminum, titanium or plastic. A number of straps work with the frame to balance the weight and hold it in place. The internal frame lets the pack fit closely to the wearer's back, which is good when the wearer is doing something that involves upper-body movement.  However, the tight fit makes it hard for the skin to breathe, so the user's back often becomes sweaty.  The internal design also makes a large storage compartment.  Lash points are few, and since the frame is integrated, it is hard to securely attach larger and heavier items to the outside.  Internal frame packs originally had smaller load carrying ability and less comfortable fit during walking, but 21st century ones are better.  In addition, because of their fit, the better internal frame models have replaced external frame backpacks for many activities.

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