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A ZIP code is the postal code used by the United States Postal Service (USPS), which always writes ZIP with capital letters. ZIP is an acronym for the Zone Improvement Plan but was also meant to suggest that mail travels more efficiently (and therefore faster) when senders use it. The basic format consists of five numerical digits. An extended ZIP+4 code includes the five digits of the ZIP code, a hyphen and then four more digits, which allow a piece of to be directed to a more precise location than by the ZIP code alone.