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Census tracts have well known/exact boundaries. ZIP codes are groups of lines whose exact structural definition is not officially established.

When looking at postal codes around the world, there are cases where two different cities are served by the same postal code. For example, the zip code 94608 in California is used for both Emeryville and parts of Oakland. One postal code, two cities.

The Postal Service designates one preferred place name for each ZIP Code. This may be an incorporated town or city, a subentity of a large city or an unincorporated census-designated place, or a small unincorporated community. Additional place names may be recognized as acceptable for a certain ZIP Code.

The first six crosswalk files are used to allocate ZIP codes to Census Bureau geographies such as census tracts, counties, county subdivisions, Core Based Statistical Areas (CBSAs), CBSA Divisions, and Congressional Districts. The last six are used to allocate from those same Census Bureau geographies to ZIP Codes.

Marketing. The data is often used in direct mail marketing campaigns in a process called ZIP-code marketing.

The first six crosswalk files are used to allocate ZIP codes to Census Bureau geographies such as census tracts, counties, county subdivisions, Core Based Statistical Areas (CBSAs), CBSA Divisions, and Congressional Districts. The last six are used to allocate from those same Census Bureau geographies to ZIP Codes.

It is preferred that all City, State, and ZIP Code information be on a single line. If that is not possible, the ZIP Code, including the ZIP+4 Code, may be placed below the city/state information.

The boundaries of a ZIP code generally have nothing to do with city limits. ZIP boundaries are set to aid mail delivery. As a result, the USPS may refer to an area by a name which may be inside an incorporated city's city limits that uses another name and still have other names that are used in common conversation.

Each ZIP Code is designated with a ?classification? indicator. Areas served only by PO Boxes receive a ?P? classification. Is there a list of ZIP Codes where the USPS doesn't deliver to physical addresses?

ZIP codes have nothing to do with the lines that define a county. One ZIP code could include addresses in multiple counties and a single county might include hundreds of ZIPs!