The concentric-zone theory is a theory of urban growth that sees growth in terms of a series of rings radiating from the central business district. Developed by Ernest Burgess in the 1920s, this model suggests that cities expand outward from a central point in a pattern of concentric zones, each with distinct social and economic characteristics. The innermost zone is typically the central business district, followed by a zone of transition characterised by mixed residential and commercial uses and often experiencing social problems. Subsequent zones include working-class housing, middle-class residential areas, and finally, the commuter zone in the outer suburbs. While this model has been influential, it has also been criticised for its oversimplification of urban patterns and its limited applicability to cities outside of the early 20th-century American context.