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.
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 ( zipping along ) 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 reference a more specific location.