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. 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.