The ZIP+4 Code is a recent innovation (introduced in late 1983) designed to direct mail to a more specific locality than the 5-digit ZIP Code provides. Essentially, the ZIP+4 Code is a subdivision of the ZIP Code geographic area.
In metropolitan areas, a ZIP+4 Code is normally assigned to each side of a street, between intersections — this is also called the block front. In addition, ZIP+4 Codes are often assigned to a single PO Box or a group of PO Boxes, to a single building, to a single apartment or a range of apartments, or to a single firm, when the volume of mail warrants such an assignment. Every mailable address in the country is assigned to at least one ZIP+4 Code. And sometimes there is both a block-face and a building code, with or without a firm code, all available to an address. In this case, all are correct, but one is finer (smaller and more specific) than the others.
In non-metropolitan areas, boundaries for ZIP+4 Codes are not always so clearly defined. Nevertheless, they still are smaller subdivisions of a 5-digit postal zone and represent a great deal of geographic specificity useful for cross-referencing census geography.
