In geography, a theory concerning the size and distribution of central places (settlements) within a system or region. The primary purpose of a settlement or market town, according to central-place theory, is the provision of goods and services for the surrounding market area. Such towns are centrally located and may be called central places. As applied to archaeology, the theory states that human settlements will space themselves evenly across a landscape as a function of the availability of natural resources, communication and transportation routes, and other factors. Eventually, these will evolve into a hierarchy of settlements of different size that depend on one another. Central-place theory attempts to illustrate how settlements locate in relation to one another, the amount of market area (goods and services) a central place can control, and why some central places function as hamlets, villages, towns, or cities. The theory was first developed by German geographer Walter Christaller. Christaller's theory concentrated on centers of different order, since in a complex system there will be some larger centers offering more specialized services to a wider area; there may indeed be many levels of such centers in a complex settlement hierarchy. Christaller's model has been modified by other geographers, especially August Losch. The theory may suggest ways in which the factors have affected the settlement pattern. Central place theory has found useful applications in archaeology as a preliminary heuristic device.