US Census TIGER/Line 2010:
ZIP Code Boundaries 

ZIP codes are a system of postal codes used by the United States Postal Service (USPS) since 1963. The term ZIP, an acronym for Zone Improvement Plan, is properly written in capital letters and was chosen to suggest that the mail travels more efficiently, and therefore more quickly, when senders use the code in the postal address. The basic format consists of five decimal numerical digits.
An extended ZIP+4 code, introduced in the 1980s, includes the five digits of the ZIP code, a hyphen, and four more digits that determine a more precise location than the ZIP code alone. Visit Wikipedia for more information about ZIP Codes.

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