Twin cities are often separated by a river - cities without this physical barrier more often become a single entity, as with the growth of the early town of London into its surrounding towns.
Some twin cities form on opposite sides of natural or governmental boundaries as conduits for trade between the two sides. For instance, Albury and Wodonga in south-eastern Australia are on the state border between New South Wales and Victoria), and formed as customs posts when the two states were independent colonies. The border between the United States and Mexico is significant in this respect because there is a chain of twin cities, particularly around the Rio Grande valley. Others began as distinct cities, but growth caused them to merge into each other and assume a single identity; an example of this is Budapest.
Note that not all geographically close cities are combined in this way. In the United Kingdom, for example, the cities of Leeds and Bradford are very close, but have strong separate identities and would not see themselves as part of the same entity.
Examples of twin cities:
The term for a large collection of cities into a single socioeconomic area is Megalopolis[?].
Contrast Twin Cities to the term twin towns, a pairing of geographically separate towns (usually international).
Compare with the term Quad Cities, which refers to a similar group of four towns. Perhaps the most famous of these are the towns of Davenport, Iowa - Bettendorf, Iowa - Rock Island, Illinois - East Moline, Illinois, all in the United States. Of these, the Iowa and Illinois towns are separated by the Mississippi River.