Borders are geographic boundaries of political entities or legal jurisdictions, such as governments, sovereign states, federated states, and other subnational entities. Borders are established through agreements between political or social entities that control those areas; the creation of these agreements is called boundary delimitation.
Some borders--such as a state's internal administrative border, or inter-state borders within the Schengen Area--are often open and completely unguarded. Other borders are partially or fully controlled, and may be crossed legally only at designated border checkpoints and border zones may be controlled.
Borders may even foster the setting up of buffer zones. A difference has also been established in academic scholarship between border and frontier, the latter denoting a state of mind rather than state boundaries.
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, there the outlaws often found sympathizers. 
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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-Kuwaiti neutral zone from 1922 until 1970. In modern times, marchlands have been replaced by clearly defined and demarcated borders. 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.
Some borders require presentation of legal paperwork like passports and visas, or other identity documents, for persons to cross borders. To stay or work within a country's borders aliens (foreign persons) may need special immigration documents or permits; but possession of such documents does not guarantee that the person should be allowed to cross the border.
Moving goods across a border often requires the payment of excise tax, often collected by customs officials. Animals (and occasionally humans) moving across borders may need to go into quarantine to prevent the spread of exotic infectious diseases. Most countries prohibit carrying illegal drugs or endangered animals across their borders. Moving goods, animals, or people illegally across a border, without declaring them or seeking permission, or deliberately evading official inspection, constitutes smuggling. Controls on car liability insurance validity and other formalities may also take place.
In places where smuggling, migration, and infiltration are a problem, many countries fortify borders with fences and barriers, and institute formal border control procedures. These can extend inland, as in the United States where the U.S. Customs and Border Protection service has jurisdiction to operate up to 100 miles from any land or sea boundary. On the other hand, some borders are merely signposted. This is common in countries within the European Schengen Area and on rural sections of the Canada-United States border. Borders may even be completely unmarked, typically in remote or forested regions; such borders are often described as "porous". Migration within territorial borders, and outside of them, represented an old and established pattern of movement in African countries, in seeking work and food, and to maintain ties with kin who had moved across the previously porous borders of their homelands. When the colonial frontiers were drawn, Western countries attempted to obtain a monopoly on the recruitment of labor in many African countries, which altered the practical and institutional context in which the old migration patterns had been followed, and some might argue, are still followed today. The frontiers were particularly porous for the physical movement of migrants, and people living in borderlands easily maintained transnational cultural and social networks.
A border may have been:
In addition, a border may be a de facto military ceasefire line.
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.
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.
Some geographical features that often constitute natural borders are:
Throughout history, technological advances have reduced the costs of transport and communication across the natural borders. That has reduced the significance of natural borders over time. As a result, political borders that have been formalized more recently, such as those in Africa or Americas, typically conform less to natural borders than very old borders, such as those in Europe or Asia, do.
Geometric boundaries are formed by straight lines (such as lines of latitude or longitude), or occasionally arcs (Pennsylvania/Delaware), regardless of the physical and cultural features of the area. Such political boundaries are often found around the states that developed out of colonial holdings, such as in North America, Africa and the Middle East. The Canada-United States border from Lake of the Woods Ontario/Minnesota to the Pacific Ocean follows the 49th parallel for roughly 2,175 miles (3,500 km).
A generalization of the idea of geometric borders is the idea of fiat boundaries by which is meant any sort of boundary that does not track an underlying bona fide physical discontinuity (fiat, latin for "let it be done", a decision). Fiat boundaries are typically the product of human demarcation, such as in demarcating electoral districts or postal districts.
A relict border is a former boundary, which may no longer be a legal boundary at all. However, the former presence of the boundary can still be seen in the landscape. For instance, the boundary between East and West Germany is no longer an international boundary, but it can still be seen because of historical markers on the landscape, and it is still a cultural and economic division in Germany.
A line of control (LoC) refers to a militarized buffer border between two or more nations that is yet to be resolved or implemented for permanent border status. LoC borders are under military control and are not recognized as an official international border. Formally known as a cease-fire line an LoC was first created as a way to buffer war borders during the Simla Agreement. Similar to a cease-fire line an LoC is the result of war, stalemates and land ownership conflict.
A maritime border is a division enclosing an area in the ocean where a nation has exclusive rights over the mineral and biological resources, encompassing maritime features, limits and zones. Maritime borders represent the jurisdictional borders of a maritime nation and are recognized by the United Nations Convention on the Law of the Sea.
Maritime borders exist in the context of territorial waters, contiguous zones, and exclusive economic zones; however, the terminology does not encompass lake or river boundaries, which are considered within the context of land boundaries.
Some maritime borders have remained indeterminate despite efforts to clarify them. This is explained by an array of factors, some of which illustrate regional problems.
Airspace is the atmosphere located within a countries controlled international and maritime borders. All sovereign countries hold the right to regulate and protect air space under the international law of Air sovereignty. The horizontal boundaries of airspace are similar to the policies of "high seas" in maritime law. Airspace extends 12 nautical miles from the coast of a country and it holds responsibility for protecting its own airspace unless under NATO peacetime protection. With international agreement a country can assume the responsibility of protecting or controlling the atmosphere over International Airspaces such as the Pacific Ocean. The vertical boundaries of airspace are not officially set or regulated internationally. However, there is a general agreement of vertical airspace ending at the point of the Kármán line. The Kármán line is a peak point at the altitude of 62 mi (100 km) above the earths surface, setting a boundary between the earths atmosphere and outer space. Airspace regulations are set by nations and local governments within an airspace, regulations of airspace differ by country and location.
An open border is the deregulation and or lack of regulation on the movement of persons between nations and jurisdictions, this does not apply to trade or movement between privately owned land areas. Most nations have open borders for travel within their nation of travel, examples of this include the United States and Canada. However, only a handful of nations have deregulated open borders with other nations, an example of this being all European Union members under the Schengen Agreement. Open borders used to be very common amongst all nations. However post WW1 lead to the regulation of open borders, making them less common and no longer feasible for most industrialized nations.
Regulated Borders have varying degrees of control on the movement of persons and trade between nations and jurisdictions. Most Industrialized nations have regulations on entry and require one or more of the following procedures: visa check, passport check or customs checks. Most regulated borders have regulations on immigration, types of wildlife and plants, and illegal objects such as drugs or weapons. Overall border regulations are placed by national and local governments and can vary depending on nation and current political or economic conditions. Some of the most regulated borders in the world include: Australia, the United States, Israel, Canada, the United Kingdom, and the United Arab Emirates. These nations have government controlled border agencies and organizations that enforce border regulation policies on and within their borders.
A demilitarized zone (DMZ) is a border separating two or more nations, groups or militaries that have agreed to prohibit the use of military activity or force within the borders bounds. A DMZ can act as a war boundary, ceasefire line, wildlife preserve, or a de facto international border. An example of a demilitarized international border is the 38th parallel between North and South Korea. Other notable DMZ zones include Antarctica and outer space (consisting of all space 100 miles away from the earth's surface), both are preserved for world research and exploration.The prohibition of control by nations can make a DMZ unexposed to human influence and thus developed into a natural border or wildlife preserve, such as on the Korean Demilitarized Zone, the Vietnamese Demilitarized Zone, and the Green Line in Cyprus.
The presence of borders often fosters certain economic features or anomalies. Wherever two jurisdictions come into contact, special economic opportunities arise for border trade. Smuggling provides a classic case; contrariwise, a border region may flourish on the provision of excise or of import-export services -- legal or quasi-legal, corrupt or legitimate. Different regulations on either side of a border may encourage services to position themselves at or near that border: thus the provision of pornography, of prostitution, of alcohol, fireworks, and/or of narcotics may cluster around borders, city limits, county lines, ports and airports. In a more planned and official context, Special Economic Zones (SEZs) often tend to cluster near borders or ports.
Even if the goods are not perceived to be undesirable, states will still seek to document and regulate the cross-border trade in order to collect tariffs and benefit from foreign currency exchange revenues. Thus, there is the concept unofficial trade in goods otherwise legal; for example, the cross-border trade in livestock by pastoralists in the Horn of Africa. Ethiopia sells an estimated $250 to $300 million of livestock to Somalia, Kenya and Djibouti every year unofficially, over 100 times the official estimate.
Human economic traffic across borders (apart from kidnapping) may involve mass commuting between workplaces and residential settlements. The removal of internal barriers to commerce, as in France after the French Revolution or in Europe since the 1940s, de-emphasises border-based economic activity and fosters free trade. Euroregions are similar official structures built around commuting across boundary.
In much of Europe, controls on persons were abolished by the 1985 Schengen Agreement and subsequent European Union legislation. Since the Treaty of Amsterdam, the competence to pass laws on crossing internal and external borders within the European Union and the associated Schengen Area states (Iceland, Norway, Switzerland, and Liechtenstein) lies exclusively within the jurisdiction of the European Union, except where states have used a specific right to opt out (United Kingdom and Ireland, which maintain the Common Travel Area amongst themselves).
The United States has notably increased measures taken in border control on the Canada-United States border and the United States-Mexico border during its War on Terrorism (See Shantz 2010). One American writer has said that the 3,600 km (2,200 mi) US-Mexico border is probably "the world's longest boundary between a First World and Third World country".[needs update]
Historic borders such as the Great Wall of China, the Maginot Line, and Hadrian's Wall have played a great many roles and been marked in different ways. While the stone walls, the Great Wall of China and the Roman Hadrian's Wall in Britain had military functions, the entirety of the Roman borders were very porous, which encouraged Roman economic activity with neighbors. On the other hand, a border like the Maginot Line was entirely military and was meant to prevent any access in what was to be World War II to France by its neighbor, Germany; Germany ended up going around the Maginot Line through Belgium just as it had done in World War I.
Macro-regional integration initiatives, such as the European Union and NAFTA, have spurred the establishment of cross-border regions. These are initiatives driven by local or regional authorities, aimed at dealing with local border-transcending problems such as transport and environmental degradation. Many cross-border regions are also active in encouraging intercultural communication and dialogue as well as cross-border economic development strategies.
In Europe, the European Union provides financial support to cross-border regions via its Interreg programme. The Council of Europe has issued the Outline Convention on Transfrontier Co-operation, providing a legal framework for cross-border co-operation even though it is in practice rarely used by Euroregions.
There has been a renaissance in the study of borders during the past two decades,[when?] partially from the creation of [clarify] Examples of recent initiatives are the Border Regions in Transition network of scholars, the International Boundaries Research Unit at the University of Durham, the Association of Borderlands Studies based in North America, the African Borderlands Research Network (ABORNE) and the founding of smaller border research centres at Nijmegen and Queen's University Belfast.
The following pictures show in how many different ways international and regional borders can be closed off, monitored, at least marked as such, or simply unremarkable.
The border between the Netherlands (right) and Germany (left) is located in the center of this residential road, and, nowadays, completely unmarked.
Italy/Switzerland border stone at Passo San Giacomo. Some borders were broadly defined by treaty, and surveyors would then choose a suitable line on the ground.
Media related to Border at Wikimedia Commons