ZIP codes

ZIP Codes are a system of postal codes used by the United States Postal Service since 1963. The term ZIP, an acronym for Zone Improvement Plan, was chosen to suggest that the mail travels more efficiently, and therefore more quickly (), when senders use the code in the postal address. The basic format consists of five numerical digits. An extended 'ZIP+4' code, introduced in 1983, includes the five digits of the ZIP Code, a hyphen, and four additional digits that determine a more specific location within a given ZIP Code.

The term ZIP Code was originally registered as a servicemark by the U.S. Postal Service, but its registration has since expired. USPS style for ZIP is all caps and the "c" in code is also capitalized, although style sheets for some publications use sentence case or lowercase.

The early history and context of postal codes began with postal district/zone numbers. The United States Post Office Department (USPOD) implemented postal zones for numerous large cities in 1943. For example:

Mr. John Smith

3256 Epiphenomenal Avenue

The "16" was the number of the postal zone within the specific city.

By the early 1960s a more organized system was needed, and on July 1, 1963, non-mandatory five-digit ZIP Codes were introduced nationwide. Three months later, on October 1, 1963, the U.S. Department of the Post Office issued its Publication 59: Abbreviations for Use with ZIP Code, with the list of two-letter state abbreviations, which are generally written with both letters capitalized. An earlier list in June had proposed capitalized abbreviations ranging from two to five letters. According to Publication 59, the two-letter standard was "based on a maximum 23-position line, because this has been found to be the most universally acceptable line capacity basis for major addressing systems", which would be exceeded by a long city name combined with a multi-letter state abbreviation, such as "Sacramento, Calif." along with the ZIP Code. The abbreviations have remained unchanged, with one exception. According to the historian of the U.S. Postal Service, "In November 1969, at the request of the Canadian postal administration, the abbreviation for Nebraska, originally NB, was changed to NE, to avoid confusion with New Brunswick in Canada".

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