An address's ZIP code and the "city" name written on the same line do not necessarily mean that that address is within that city. The Postal Service designates a single "default" place name for each ZIP code. This may be an actual incorporated town or city, a sub-entity of a town or city or an unincorporated census-designated place. Additional place names, also of any of these types, may be recognized as "acceptable" for a certain ZIP code. Still others are deemed "not acceptable", and if used, may result in a delay in mail delivery.
Default place names are typically the actual city or town in which the address is located. However, for many cities that have incorporated since ZIP codes were introduced, the actual city name is only "acceptable" and not the "default" place name. Many databases automatically assign the "default" place name for a ZIP code, without regard to any "acceptable" place names. For example, Centennial, Colorado, the largest city to incorporate in U.S. history, is divided among seven ZIP codes assigned to "Aurora", "Englewood" or "Littleton" as its "default" place names. Thus, postally speaking, the city of Centennial and its 100,000 residents do not exist - they are part of Aurora, Englewood or Littleton. In the ZIP code directory, Centennial addresses are listed under those three cities. And since it is "acceptable" to write "Centennial" in conjunction with any of the seven ZIP codes, one can write "Centennial" in an address that is actually in Aurora, Englewood, or Littleton, as long as it is in one of the shared ZIP codes.
"Acceptable" place names are often added to a ZIP code in cases where the ZIP code boundaries divide them between two or more cities, as in the case of Centennial. However, in many cases only the "default" name can be used, even when many addresses in the ZIP code are in another city. For example, approximately 85% of the area served by the ZIP code 85254, to which the place name "Scottsdale, Arizona," is assigned, is actually inside the city limits of neighboring Phoenix. This is because the post office that serves this area is in Scottsdale. This has led some residents of the ZIP code to believe that they live in Scottsdale when they actually live in Phoenix. A City of Scottsdale Web site listing the positive and negative aspects of the city mentioned the 85254 ZIP code as a positive aspect because "Scottsdale" is being used for businesses located outside the city limits in Phoenix.
This phenomenon is repeated across the country. The previously mentioned Englewood is a land-locked, inner-ring suburb that was built out by the 1960s. Its post office served the area that is now the high-growth southern tier of the Denver metropolitan area, and ZIP codes in this area were assigned "Englewood" as their "default" place name. An employment center as large as downtown Denver has grown in this area, and its office parks are the headquarters for many internationally recognized corporations. Even though they are actually located in other cities, they indicate "Englewood" as their location, as this is the "default" postal place name. As a result, there are really two "Englewoods" — the actual city, small and with a largely working-class residential population; and, a number of miles away, the postal "Englewood," a vast suburban area of upscale subdivisions and office parks that have nothing to do with the City of Englewood yet share a split identity with it solely because of ZIP codes. People who say that they live or work in "Englewood" and identify closely with it may rarely enter the actual city of that name.
"Acceptable" place names also come into play in areas of the country where many citizens identify more strongly with a particular urban center than the municipality in which they actually live. For example, Allegheny County, Pennsylvania has 130 distinct municipalities, but many of the county's residents, and even some residents of adjacent counties, commonly use "Pittsburgh, PA" as their postal address.
Finally, many ZIP codes are for villages, census-designated places, portions of cities, or other entities that are not municipalities. For example, ZIP code 03750 is for Etna, New Hampshire, but Etna is not a city or town; it is actually a village district in the town of Hanover, which itself is assigned the ZIP code 03755. Another example is ZIP code 08043, which corresponds to the census-designated place of Kirkwood, NJ but actually serves the entirety of Voorhees Township, NJ.
The postal designations for place names become de facto locations for their addresses, and as a result it is difficult to convince residents and businesses that they actually are located in another city or town different from the "default" place name associated with their ZIP codes. Because of the confusion and lack of identity generated by this situation, some cities, such as Signal Hill, California, have successfully petitioned the Postal Service to change ZIP code boundaries or create new ZIP codes so that their cities can be the "default" place name for addresses within the ZIP code.
This confusion also can have financial implications for local governments, because mail volume is among the factors used by the US Census to estimate population changes between decennial census enumerations. Sometimes local officials in a community that is not the "default" place name for a zip code but is an "acceptable" place name will advise residents to always use the name of the community, because if the census estimate of that town's population is low they will get less of various state and federal funds that are computed based on population.