Postal zone

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

The term ZIP Code was originally registered as a servicemark by the U.S. Postal Service, but its registration has since expired.

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 from the specific city.

By the early 1960s, a more organized system was needed, and non-mandatory five-digit ZIP Codes were introduced nationwide on July 1, 1963. The USPOD issued its Publication 59: Abbreviations for Use with ZIP Code on October 1, 1963, 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 the exception of Nebraska, which was changed from NB to NE in 1969 at the request of the Canadian postal administration, to avoid confusion with the Canadian province of New Brunswick.

...
Wikipedia