Twin cities

Twin cities are a special case of two neighboring cities or urban centres that grow into a single conurbation – or narrowly separated urban areas – over time. 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 phenomena involving 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 (New South Wales) and Wodonga (Victoria) in Australia, on opposite sides of the Murray River. Islamabad and Rawalpindi are the twin cities of Pakistan; Islamabad is the Federal Territory of Pakistan, while Rawalpindi is a city in the Punjab province. Cities on opposite sides of international borders sometimes share enough cultural and historical identity to be seen as twins, such as Haparanda (Sweden) and Tornio (Finland), Leticia (Colombia) and Tabatinga (Brazil), or Valga (Estonia) and Valka (Latvia).

In some cases twin cities eventually merge into a single legal municipality, such as Buda and Pest merging in 1873 into Budapest, Hungary; Brooklyn being annexed by New York City in 1898; or the three ancient cities of Hankou, Hanyang, and Wuchang joining in 1927 into Wuhan, China.

As a single urban area, twin cities may share an airport whose airport codes include both cities' initials, e.g., DFW (Dallas–Fort Worth), LBA (Leeds–Bradford), MSP (Minneapolis–Saint Paul), RDU (Raleigh and Durham), and CAK (Akron–Canton).