Source: adapted from World Bank (2009) World Development Report 2009: Reshaping Economic Geography.
Conventional perspectives about the urban spatial structure usually represent the urban and rural spaces as a dichotomy. They were considered two separate entities, albeit interacting. However, the urban spatial structure is better understood as a continuum composed of a variety of transitional structures between what can be considered purely rural and urban. The firsts are hamlets and villages representing basic forms of urbanism in a rural setting. Then, a whole range of urban settlements ranging from towns, cities, and large urban agglomerations such as metropolises. At the top of the hierarchy is the megacity of usually more than 10 million people, most of the time ranking among global cities.
There is also a regionalism to urban areas, implying that they can be grouped into larger functional entities. For instance, the Extended Metropolitan Region (EMR; often labeled a metropolis) is a continuum of urban activities, often interwoven with rural activities, that includes a large urban agglomeration and a network of secondary (satellite) cities. At a higher level, megalopolises (also labeled as mega-urban regions) are massive urban conurbations often encompassing several EMRs structured along a corridor, such as BostWash (Boston – Washington), Tokaido (Tokyo – Osaka), or the Pearl River Delta (Hong Kong – Guangzhou) or Shanghai – Nanjing.