A suburb is a mixed-use or residential area, existing either as part of a city or urban area or as a separate residential community within commuting distance of a city. Suburbs might have their political jurisdiction, especially in the United States, but this is not always the case, especially in the United Kingdom where suburbs are located within the administrative boundaries of cities. In most English-speaking countries, suburban areas are defined in contrast to central or inner-city areas, but in Australian English and South African English, suburb has become largely synonymous with what is called a "neighborhood" in other countries and the term extends to inner-city areas. In some areas, such as Australia, India, China, New Zealand, the United Kingdom, and parts of the United States and Canada, new suburbs are routinely annexed by adjacent cities. In others, such as Morocco, France, and much of the United States and Canada, many suburbs remain separate municipalities or are governed as part of a larger local government area such as a county. In the United States, beyond the suburbs are exurbs, or "exurban areas", with less density but linked to the metropolitan area economically and by commuters. Suburbs first emerged on a large scale in the 19th and 20th centuries as a result of improved rail and road transport, which led to an increase in commuting. In general, they have lower population densities than inner city neighborhoods within a metropolitan area, and most residents commute to central cities or other business districts; however, there are many exceptions, including industrial suburbs, planned communities, and satellite cities. Suburbs tend to proliferate around cities that have an abundance of adjacent flat land.