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 There are incorporated places, particularly in the Northeast, that have not had a boundary change this century; there are a few places in Alabama and California that have, in recent years, had boundary changes virtually every month.

There is only a limited relationship between place size and place population, and the relationship seems to vary by region. The most densely settled places generally are the older cities in the Northeast region, cities that underwent early development and tend to have relatively fixed boundaries. In the Northeastern States, the MCDs have strong governments that often have all the powers associated with incorporated places; as a consequence, annexation for the purpose of providing municipal services is unnecessary, and in some States is difficult, if not impossible. In some Midwestern and Southern States, boundary change laws are more permissive, and aggressive or widespread annexations often result in lower population densities for places.

Recognizing the effects of city/county consolidations and unrestricted annexation practices in some States, the Census Bureau developed the extended city concept for the 1970 census. This concept modified the urban and rural classifications by defining, within UAs, certain sparsely settled portions of large-area incorporated places as rural. In 1980, after identifying extended cities in UAs, there still were nine sparsely settled incorporated places outside UAs that contained almost 7,700 square miles of territory, an area larger than the State of New Jersey. This distorted the national percentage of urban area by nearly 10 percent. To correct this situation for 1990, the Census Bureau modified the extended city criteria to include non-UA incorporated places. (For further information on extended cities, both inside and outside of UAs, see .)

Incorporated place boundaries are subject to change; in some States, many do so frequently. The instruments of change are municipal annexation and detachment, merger or consolidation, and incorporation and disincorporation. Beginning in 1970, the Census Bureau recognized boundaries legally in effect on January 1 of the census year, rather than April 1 (census day) 9-14Places