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Definitions {{ safesubst:#invoke:Unsubst||$N=Unreferenced section|date=__DATE__ |$B= {{#invoke:Message box|ambox}} }}

The purpose of the Great Wall of China was to stop people from crossing the northern border of China.

In the past, many borders were not clearly defined lines, but were neutral zones called marchlands. This has been reflected in recent times with the neutral zones that were set up along part of Saudi Arabia's borders with Kuwait and Iraq (however, these zones no longer exist){{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }}. In modern times, the concept of a marchland has been replaced by that of the clearly defined and demarcated border. 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, plants, and goods into or out of the country. Under international law, each country is generally permitted to define the conditions that have to be met by a person to legally cross its borders by its own laws, and to prevent persons from crossing its border when this happens in violation of those laws.

Some borders require presentation of legal paperwork like passports and visas, or other identity documents to cross borders. To stay or work within a country's borders aliens (foreign persons) may need special immigration documents or permits that authorise them to do so. Having such documents (i.e., visa and passport) however does not automatically guarantee that the alien will be allowed to cross to the other side of 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 or 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, seeking permission, or deliberately evading official inspection constitutes smuggling.

In regions where smuggling, migration, and infiltration are a problem, many countries fortify borders with fences and barriers and institute formal border control procedures. Some borders are only 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 by people 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 from the point of view of the physical movement of migrants, and people living in borderland communities easily maintained transnational cultural and social networks.

A border may have been:

  • Agreed by the countries on both sides
  • Imposed by the country on one side
  • Imposed by third parties, e.g. an international conference
  • Inherited from a former state, colonial power or aristocratic territory
  • Inherited from a former internal border, such as within the former Soviet Union
  • Never formally defined.

In addition, a border may be a de facto military ceasefire line.

Border sections
Intro  Definitions  Classification of borders  Maritime borders  Border economics  Politics  Cross-border regions  Border studies  Image gallery  See also  References  Further reading  External links  

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