ZIP Codes 101

An extended ZIP+4 code includes the five digits of the ZIP Code plus four more digits which allow a piece of mail to be even more accurately directed to a very small geography. In 1983 the U.S. Postal Service began using the expanded ZIP (Zone Improvement Plan) code in order to expedite shipping to specific office blocks, mailing centers, and other areas that receive a high volume of mail. The last four numbers aren't mandatory, but they can help expedite delivery. You can find the "ZIP + 4" of any address in the United States by using this tool from the Postal Service. If you're using a ZIP+4, always remember to separate the two numbers with a hyphen. There is not a uniform rule for ZIP+4 codes so it may require looking them up individually for each box.

The USPS also explains how ZIP codes break down: the first three numbers indicate a "sectional center" or large city, the second two indicate the specific post office facility, the third two indicate a sector or several blocks, and the last two indicate a segment or side of a street. ZIP codes start with "0" on the east coast and increase westwards across the continent (hence "Beverly Hills 90210").

ZIP Code was originally registered as a trademark by the U.S. Postal Service, but its registration has since expired. The postal service implemented postal zones for large cities in 1943. By the early 1960's a more general system was needed, and on July 1, 1963, non-mandatory ZIP Codes were announced for the whole country. Robert Moon, an employee of the post office, is considered the father of the ZIP Code. He first submitted his proposal in 1944 while working as a postal inspector. The post office only gives credit to Moon for the first 3 digits of the ZIP Code, which describe the region of the country. In 1967, these were made mandatory for second- and third-class bulk mailers and the system was soon adopted generally. The United States Post Office used a cartoon character, Mr. ZIP, to promote use of the ZIP Code in the 1970s.

Taken from Wiki & Ask Yahoo