ZIP codes are a zip code system used by the U.S. Postal Service since the early 1960s. ZIP codes are used to sort mail faster and speed up delivery. They were originally made up of five digits, but were expanded in the 1980s to nine digits, which are written with a hyphen - XXXXX-YYYYY, such as 98765-4321.
The U.S. Postal Service began using zip codes in large cities as early as 1943, when the mailing address began to be written like this:
Mr. Robert Smith
3256 Epiphenomenal Avenue.
Minneapolis 16, Minnesota.
The number 16 in this case refers to the number of the postal district within the city. By the early 1960s there was a need for a more comprehensive postal code system. On July 1, 1963 this new system was adopted in test mode throughout the United States. The creator of the ZIP codes is believed to be Robert Moon, who worked for the Post Office.
In 1983 the U.S. Postal Service began using the ZIP + 4 indexing system, which is based on an expanded postal code consisting of nine digits. Four more digits were added (through a hyphen) to the five-digit ZIP code that was already in use. These digits denote a local geographic segment within the five-digit delivery area, such as a city block, an apartment complex, an individual recipient sending large volumes of mail, or any division of an organization that requires an additional identifier to facilitate mail sorting and delivery. U.S. government agencies have separate five-digit and nine-digit ZIP codes that do not provide for other postal service users in the postal area with a given zip code. Occasionally, ZIP codes for an area may be revised or eliminated altogether.