Barbed wire is a type of wire constructed with sharp edges or points. It is used to construct inexpensive fences. When a person or other animal tries to pass through a barbed-wire fence they will suffer discomfort, or possibly injury. Barbed wire fencing requires only fenceposts and wire, and is simple to construct and quick to erect.
Barbed wire was invented by Joseph F. Glidden of Dekalb, Illinois in 1873. It was an improvement on less successful pointed wire products invented in 1867-68.
Barbed wire was the first wire technology capable of restraining cattle. Wire fences were cheaper to erect than their predecessors, and when they became widely available in the late 19th century in the United States they made it affordable to fence much bigger areas than before. They made intensive animal husbandry practical on a much larger scale.
In the American Southwest barbed-wire fencing led to disputes known as the range wars between free range ranchers and farmers in the late-19th century. These were similar to the disputes which resulted from enclosure laws in England in the early 18th century. These disputes were decisively settled in favor of the farmers, and heavy penalities were instituted for cutting the wire in a barbed wire fence.
Barbed wire fences remain the standard fencing technology for cattle in most regions. The wire is suspended under tension between heavy, braced, fence posts, and then aligned by being attached to steel star posts. The gaps between star posts vary depending on terrain - on short fences in hilly country they may be placed as closely as every 3 metres, whereas in flat terrain with long spans and relatively few stock they may be spaced out up to 30-50 metres.
Barbed wire for agricultural fencing is typically available in two varieties - "soft" wire and "high tensile". High-tensile wire is made with thinner but higher-strength and more elastic steel. Its greater elasticity make its fences longer-lasting. It copes with the expansions and contractions caused by heat and animal pressure by stretching and relaxing within wider elastic limits. It also supports longer spans, but because of its "springy" nature it is unpleasant and somewhat dangerous for inexperienced fencers. Soft wire is much easier to work, but is less durable and only suitable for short spans.
Most barbed wire fences, while sufficient to discourage cattle, are passable by humans who can simply climb through the fence by stretching the gaps between the wires, using non-barbed sections of the wire as handholds.
To prevent human crossing, many prisons and other high-security installations construct fences with razor wire, a variant which instead of occasional barbs features near-continuous cutting surfaces sufficient to severely injure anyone who climbs over it.