The area code is the system of postal codes used by the United States Postal Service (USPS). The letters ZIP, an acronym for Zone Improvement Plan,are properly written in capital letters and were chosen to suggest that the mail travels more efficiently, and therefore more quickly, when senders use the code. The basic format consists of five numerical digits.An extended ZIP + 4 code 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. The term ZIP Code was originally registered as a servicemark (a type of trademark) by the U.S. Postal Service but its registration has since expired.