The ZIP code is the system of postal codes used by the United States Postal Service (USPS). The letters "ZIP" is an acronym for Zone Improvement Plan. The letters are written properly in capital letters. The word ZIP was chosen to suggest that the mail travels more efficiently, and more quickly, when senders use it. The basic ZIP code is made up of five numbers. An extended ZIP + 4 code includes the five digits of the ZIP code, a hyphen and then four more digits. This lets a piece of mail to be sent to a more exact location than by the ZIP code alone. "ZIP Code" was a trademark of the U.S. Postal Service but its registration has since ended.