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From CreationWiki, the encyclopedia of creation science

The United States Postal Service established its Zone Improvement Plan, or ZIP for short, to facilitate the easy handling of the large quantities of mail that often go to a single city or even to one post-office station that handles mail for part of a city. This is the American equivalent of the generic concept of "postal code."

Under this plan, each United States Post Office station has a unique five-digit identifier.

The first digit identifies one of ten federal regions into which the USPS divides the territory of the United States.

The second digit identifies the State or often the part of the State.

The third identifies a city or county or larger region.

The last two digits identify the unique station within the above region.

Lately the Post Office refined the ZIP and added an optional four digits--which constitute a unique identifier for a particular residence, business, or other organization that receives mail from that station.