The term twin cities refers to urban cities that are so close together that they grow into each other. In the United States, the term usually refers to Minneapolis and Saint Paul, Minnesota. This joined metropolitan area, Minneapolis-St. Paul, is one of the largest metro areas in the country and the largest in the state; the region has more than three million residents. Interestingly, this metro area doesn't really consist of just two cities; it actually encompasses more than 180 different cities and towns. However, it is named for its two largest cities, Minneapolis and Saint Paul, which have downtowns that are just 9 miles (14.48 kilometers) apart and share an airport.
The Twin Cities area accounts for more of the population of the state than any other metropolitan area in Minnesota. Minneapolis is larger than its sister, Saint Paul, but Saint Paul has the impressive distinction of being the state's capitol. The sister cities gained the name Dual Cities around 1872, and the initial intent of the nickname was to remind people of the fact that there were actually two cities instead of just one. Eventually, the named changed to Twin Cities and remains so today.
Interestingly, each of the Twin Cities maintains its own separate borders and distinct physical makeup. Minneapolis has wide streets and is said to have an easier-to-navigate layout while Saint Paul's features streets that are slimmer and more dense neighborhoods. Even the cultural makeup of the two cities is different, as Minneapolis has Scandinavian and Lutheran roots and Saint Paul has more of an influence of the Catholics, Irish, and Germans of yesteryear. The cities even differ architecturally, with Minneapolis having more modern buildings and Saint Paul leaning more towards classic and Victorian styles, but also integrating some newer types of structures.
Though you might think that the Twin Cities would be content to grow into each other, they tend to function more as rivals. Each has tried to outdo the other as far as building is concerned. Each of the Twin Cities boasts its own University of Minnesota campus, and both have elaborate places of worship. The Saint Paul Cathedral was built in 1915, and Minneapolis built its Basilica of St. Mary in 1926. Even as far back as 1890, the cities rivaled each other, going so far as to kidnap each other's census takers back in 1890.
Besides Minneapolis and Saint Paul, twin cities exist around the world, such as Vienna-Bratislava in Europe and Leticia, Colombia and Tabatinga, Brazil in South America. It's important to note that while twin cities are referred to collectively as one metro area, they often remain drastically different in terms of money, politics, demographics, and geographic makeup. While it may seem to make sense for such cities to merge together and become one officially, there typically seems to be quite a bit of resistance to doing so.