<P> 3256 Epiphenomenal Avenue </P> <P> The "16" was the number of the postal zone from the specific city . </P> <P> 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 . </P> <P> Robert Moon is considered the father of the ZIP Code; he submitted his proposal in 1944 while working as a postal inspector . The post office only credits Moon with 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 fourth and fifth digits were proposed by Henry Bentley Hahn Sr which give a more precise locale within the SCF . 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 . Sectional centers do not deliver mail and are not open to the public (though the building may include a post office open to the public), and most of their employees work night shift . Mail picked up at post offices is sent to their own SCF in the afternoon, where the mail is sorted overnight . In the cases of large cities, the last two digits coincide with the older postal zone number thus: </P>

When did the post office start using zip codes