The the middle of WW II the growing U.S. population and the increasing the amount of mail sent among them led the United States Post Office Department to divided cities into numerical zones (so, for example, one would send a letter to a street address in San Francisco, 6, California). The sorting of letters into their destination regions, however, was still done by hand. Two decades later the volume of mail got big enough to begin choking the system, and the Post Office had to develop a new model of distribution. What they came up with was the ZIP.

On this day, July 1, in 1963, the Zone Improvement Plan went into effect, with five-digit codes indicating the regional and local mail facilities through which the letter would go, were introduced. Use of the code was not mandatory on any mail at first, and only began a requirement on second- and third-class bulk mail four years later.

Robert Moon, a postal inspector, submitted his proposal for the ZIP system in 1944. His proposal had only the first three of the modern-day five-digit zip code, designating the broad geographical regions and the more localized sectional center facility where the letter would go. The final two digits indicate the actual neighborhood post offices that distribute the mail to houses.