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. In the United States, "the Twin Cities" usually refers to the Minneapolis-Saint Paul metropolitan area in Minnesota.
Cities twinned by proximity do not necessarily match demographically, economically, or politically.
In most cases, cities that grow into each other's space lose their individual identities, and the border or barrier that still separates them becomes almost irrelevant. An 1873 case of twin cities becoming united cities is Budapest in Hungary, which began as two settlements (Buda and Pest) facing each other across the Danube at a strategic fording place along a trade route.
In some cases, such as Albury/Wodonga in Australia, the two cities are permanently divided by a state border, often one that strictly adheres to a geographical landmark, such as the Murray River that divides New South Wales from Victoria, and thus Albury from Wodonga.