The modern city as seen in American and other Western cultures is characterized by a variegated pattern of land-use areas. There are sections dominated by business establishments, localities where manufacturing prevails, vast expanses that are residential, and areas of vacant land awaiting development. This is the central business district (CBD), often popularly known as downtown. The names "central business district," "central commercial district," "retail business section," "downtown business district," and several others have been used in place of "central business district," but this designation has become more and more predominant. All three concepts contribute to an understanding of the CBD. Central-place theory also applies on a different scale. There is a hierarchy of retail areas within cities. At this level the CBD is only one of the central places that can be recognized within an urban center. Central-place theory enhances our understanding of city commercial structure by enlarging the foundations provided by the concentric zone, sector, and multiple nuclei theories.