<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (March 2011) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (March 2011) (Learn how and when to remove this template message) </Td> </Tr> <P> Twin cities are a special case of two cities or urban centres that are founded in close geographic proximity and then grow into each other over time, losing most of their mutual buffer zone . </P> <P> There are no precise criteria for twin - cityhood, but to be considered twin cities, the cities involved have to have a similar administrative status and somewhat comparable sizes; a suburb of a much larger population center is usually not considered to form a twin city with it . For example, South San Francisco (population about 65,000) is not considered a twin city with San Francisco (population about 850,000). However, cities considered twinned by proximity do not necessarily match demographically, economically, or politically . </P>

Why do they call them the twin cities