Twin cities
There are no formal criteria, but twin cities are generally comparable in status and size, though not necessarily equal; a city and a substantially smaller suburb would not typically qualify, even if they were once separate. Tri-cities and quad cities are similar groups of three or four municipalities.
A common – but not universal – scenario is two cities that developed concurrently on opposite sides of a river. For example, Minneapolis and Saint Paul in Minnesota – one of the most widely known pairs of "Twin Cities" – were founded several miles apart on opposite sides of the Mississippi River, and competed for prominence as they grew.
In some cases, twin cities are separated by a state border, such as Albury and Wodonga in Australia, on opposite sides of the Murray River. In Pakistan, Islamabad and Rawalpindi are twin cities located in northwestern Punjab region: Islamabad is administratively being part of the Islamabad Capital Territory, and Rawalpindi is in the province of Punjab. Cities on opposite sides of international borders sometimes share enough cultural and historical identity to be seen as twins, such as Haparanda and Tornio, Leticia and Tabatinga, or Valga and Valka.
In some cases twin cities eventually merge into a single legal municipality, such as Buda and Pest merging in 1873 into Budapest, Hungary; Brooklyn and New York City being consolidated in 1898; and the three ancient cities of Hankou, Hanyang, and Wuchang joining in 1927 into Wuhan.
As a single urban area, twin cities may share an airport whose airport codes include both cities' initials, e.g. DFW, LBA, MSP, RDU, and CAK.
Twin cities
List of International border towns and cities
Africa
Asia
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| Astara and Astara |  AzerbaijanEuropeNorth AmericaSouth AmericaList of internal border towns and citiesAfrica
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Egypt
Denmark
Australia