A haversack is a bag with a single strap which is designed to be worn over one shoulder, or crossed across the chest if the haversack is particularly heavy. Haversacks are very useful for carrying small amounts of supplies; children might take them to school, for example, or someone might bring one to a picnic. The bags are related to backpacks, although they cannot hold as much as a regular backpack and they do not distribute their load evenly over the bearer, making them a poor choice for heavy loads.
At one point, haversacks were standard issue for many militaries. Soldiers carried a small amount of food in their haversacks, along with other vitally important supplies. Over time, haversacks were replaced by rucksacks and later backpacks. Backpacks are more suited to military use since they can be used to carry a large amount of supplies. In addition, when packed properly, a backpack will evenly distribute weight, minimizing the risk of injury.
Historical re-enactors often use haversacks for authenticity, since the bags were widely used by a range of people throughout history. Biologists sometimes used haversacks for field studies so that they have a place to quickly store collected samples. Others prefer using haversacks because they are smaller and more convenient to access then backpacks. The classic messenger bags used by bikers and postmen, for example, rely on the basic haversack design, which includes a single strong strap and a covering flap which can easily be flipped back to access the contents.
Many stores sell haversacks, or modern versions of these classic carry-all bags. It is also possible to sew your own haversack, as the design of the bag is extremely simple and easy to replicate. A hand-sewn haversack may also integrate custom pockets and flaps which are uniquely designed for the individual user. As a general rule, sturdy materials like canvas and nylon are used to make a haversack, ensuring that it doesn't fall apart in the field. Some may also be waterproofed.
When seeking out a haversack, think about how you intend to use it. If you want a haversack for use in the field, for example, you may want a waterproofed version with space to hold samples and objects that you collect. If you're taking a haversack to school, ample room for books and supplies is a necessity, whereas a historical re-enactor might want a haversack made from authentic materials. In all cases, make sure that the haversack is well sewn, and that the strap is wide enough to distribute the weight of the bag, rather than concentrating it in one place.