The US Postal Service uses zipcodes as part of the addresses so that it can direct the mail to the appropriate person. The ZIP part of the word is an acronym for Zone Improvement Plan. The basic format of all zip codes consists of five numbers, In 1983 a ZIP + 4 format was introduced, which consists of the basic five numbers, a hyphen and then four more numbers.



The process of using zip codes for pieces of mail in the US started in 1943 when the US Post Office started using codes to identify large cities within states. This was the brainchild or a postal worker, Robert Moon, who submitted his proposal to the Postal service in 1941. A more general system was introduced in the early 1960's and a zip cods for the entire country came into effect in 1963. Moon's proposal was to use three digits to identify each sectional center facility in the country and this system is still in effect. The five digits consist of the number for the sectional facility and the postal code for the city in the state. For example the zip code for Aaronsburg, Pennsylvania is 16820. The numbers 168 refer to the sectional center facility and the numbers 20 refers to the postal code for Pennsylvania.

In 1963, zip cods were only used for first class mail and it wasn't until 1967 that packages and second and third class mail were required to use them in the addresses. In the ZIP + 4, system now in use, the additional four digits are used to identify a geographic region. The code is transferred to a bar code which the sorting machinery used in the post offices can use to make sure the mail goes to the appropriate locality. For addresses with post office boxes, the add on digits refer to the post office box number. However, unlike the regular zip codes for cities, there is no uniform code for post office boxes. If you want to send mail to a particular address you would have to look up the full 12-digit zip code.

The first digit of the zip code represents a geographical area of the country, such as a group of states. Together with this digit and the next two, the mail goes to the appropriate facility where the mail is sorted once again. The fourth and fifth digits of the zip code refer to the addresses within a specific region. Usually the main city in a region receives the first zip code and then the surrounding towns and cities receive zip codes in alphabetical order following that initial zip code according to the name of the city or town.

Most of the lowest zip code numbers are in the New England states, since they all start with 0. Other postal codes beginning with 0 are the US Internal Revenue Service, Adjuntas, Puerto Rico and cities starting with A in this region. New York State has the initial digit of 1, so the zip codes of all addresses in New York start with 1.