extension of the five-digit ZIP code providing the U.S. Postal Service with even more precise destination information to facilitate mail handling. The nine-digit ZIP is based on a system of blockface grids. The first two digits of the added four digits are a sector; the last two digits are a segment within a sector.

The USPS intends the nine-digit ZIP to be used by business mailers, not by individuals, and offers a small discount for its use. Another benefit provided by the nine-digit ZIP is that it can serve as a basis for demographic and psychographic selections from mailing lists.