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 the exception that in 1969, at the request of the Canadian postal administration, the abbreviation for Nebraska was changed from NB to NE to avoid confusion with the Canadian province of New Brunswick.
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 ( zipping along ), when senders use the code in the postal address . The basic format consists of five 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.