Border

Borders are generally defined as geographical boundaries, imposed either by features such as oceans and terrain, or by political entities such as governments, sovereign states, federated states, and other subnational entities. Political borders can be established through warfare, colonization, or mutual agreements between the political entities that reside in those areas.

Some borders—such as most states' internal administrative borders, or inter-state borders within the Schengen Area—are open and completely unguarded. Most external political borders are partially or fully controlled, and may be crossed legally only at designated border checkpoints; adjacent border zones may also be controlled. For the purposes of border control, airports and seaports are also classed as borders. Most countries have some form of border control to regulate or limit the movement of people, animals, and goods into and out of the country. Under international law, each country is generally permitted to legislate the conditions that have to be met in order to cross its borders, and to prevent people from crossing its borders in violation of those laws.

Buffer zones may be set up on borders between belligerent entities to lower the risk of escalation. While border refers to the boundary itself, the area around the border is called the frontier.

In the pre-modern world, the term border was vague and could refer to either side of the boundary, thus it was necessary to specify part of it with borderline or borderland. During the medieval period the government's control frequently diminished the further people got from the capital. Therefore borderland (especially impassable terrain) attracted many outlaws, as they often found sympathizers.

In the past, many borders were not clearly defined lines; instead there were often intervening areas often claimed and fought over by both sides, sometimes called marchlands. Special cases in modern times were the Saudi Arabian–Iraqi neutral zone from 1922 to 1991 and the Saudi Arabian–Kuwaiti neutral zone from 1922 until 1970. In modern times, marchlands have been replaced by clearly defined and demarcated borders.

Political borders are imposed on the world through human agency. That means that although a political border may follow a river or mountain range, such a feature does not automatically define the political border, even though it may be a major physical barrier to crossing.

Political borders are often classified by whether or not they follow conspicuous physical features on the earth. William Miles said that Britain and France traced close to 40% of the entire length of the world's international boundaries.

Natural borders are geographical features that present natural obstacles to communication and transport. Existing political borders are often a formalization of such historical, natural obstacles.