ZIP codes are a system of postal codes used by the United States Postal Service (USPS) since 1963. An extended ZIP+4 code, introduced in the 1980s, includes the five digits of the ZIP code, a hyphen, and four more digits that determine a more precise location than the ZIP code alone.
Simultaneously with the introduction of the ZIP code, two-letter state abbreviations were introduced.  The post office gives credit to Moon for only the first three digits of the ZIP code, which describe the sectional center facility (SCF) or "sec center." An SCF is a central mail processing facility with those three digits. The SCF sorts mail to all post offices with those first three digits in their ZIP codes. The mail is sorted according to the final two digits of the ZIP code and sent to the corresponding post offices in the early morning. The United States Post Office used a cartoon character, whom it called Mr. ZIP, to promote use of the ZIP code.In 1983, the U.S. Postal Service began using an expanded ZIP code system that it called ZIP+4, often called "plus-four codes", "add-on codes", or "add ons".
A ZIP+4 code uses the basic five-digit code plus four additional digits to identify a geographic segment within the five-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery. For post-office boxes, the general (but not invariable) rule is that each box has its own ZIP+4 code. It is common to use add-on code 9998 for mail addressed to the postmaster (to which requests for pictorial cancellations are usually addressed), 9999 for general delivery and other high-numbered add-on codes for business reply mail. For a unique ZIP code (explained below), the add-on code is typically 0001.
For example ZIP code 42223 spans Christian KY and Montgomery TN, and ZIP code 97635 spans Lake OR and Modoc CA.
A sectional center facility may have more than one three-digit code assigned to it. For example, the Northern Virginia sectional center facility in Merrifield is assigned codes 220, 221, 222 and 223. The lowest ZIP code is in Holtsville, New York (00501, a ZIP code exclusively for the U.S. Internal Revenue Service center there). Other low ZIP codes are 00601 for Adjuntas, Puerto Rico; 01001 for Agawam, Massachusetts, and 01002 for Amherst, Massachusetts.
Despite the geographic derivation of most ZIP codes, the codes themselves do not represent geographic regions; in general, they correspond to address groups or delivery routes. For example, some residents in or near Haubstadt, Indiana, which has the zip code 47639, have mailing addresses with 47648, the ZIP code for neighboring Fort Branch, Indiana, while others living in or near Fort Branch have addresses with 47639. While the White House itself is located in ZIP code 20006, it has the ZIP code 20500. The Nuclear Regulatory Commission is located in Rockville, Maryland, at ZIP code 20852, but has been assigned by the Postal Service the address "Washington, DC 20555".
In rare circumstances, a locality is assigned a ZIP code that does not match the rest of the state; in other words, a ZIP code may cross state lines. Fishers Island, New York, bears the ZIP code 06390 and is served from Connecticut — all other New York ZIP codes (excepting those at Holtsville for the IRS) begin with "1".
Some Texas ZIP codes are served from New Mexico (most notably some El Paso ZIP codes) and thus bear codes beginning with "885" (contiguous numerically with 870-884 NM) rather than "799".
Fort Campbell (ZIP code 42223), primarily in Kentucky, main entrance is in Tennessee.
In essence the mail for the Kenton County Airport Board has an Ohio ZIP code but the physical address of the airport is a Kentucky ZIP code.