What is a ZIP code plus 4?

A ZIP+4 code is a nine-digit number the United States Postal Service uses to route mail to recipients. In addition to the five-digit ZIP code, it contains a four-digit identifier to further narrow down the destination of each piece of mail.

The ZIP code was originally developed to help the postal service route mail more efficiently, but as population density increased, the system began to show strain. ZIP+4 was instituted in 1983 to establish a more complete mail routing system. The ZIP+4 codes can, in some cases, direct a piece of mail all the way to its intended recipient, even identifying the destination floor of a building or department of a company.