What Is The ZIP Code?

A ZIP (Zone Improvement Plan) Code has been used by the US Postal Service since 1963.
A ZIP (Zone Improvement Plan) Code has been used by the US Postal Service since 1963.

A ZIP Code is a system that is made up of postal codes which have been used by the US Postal Service from the year 1963. ZIP is an acronym which stands for Zone Improvement Plan. When senders use the postal address code, which consists of five digits, the mail is sent more effectively and quicker, hence the name. The primary ZIP Code format comprises of five digits. In 1983, ‘ZIP +4’ an extended code was introduced; it featured the five digits, a hyphen and four more digits which were added in orders to locate the destination more precisely. The US Postal Service originally registered the term ‘ZIP Code’ as a service mark but has since expired.

History Of The ZIP Code

The context of postal codes started with zone numbers or postal district. In 1943, there was the implementation of postal codes for some large cities done by the US Post Office Department (USPOD). During the early 1960's, a more strategic system was required leading to the introduction of nonmandatory five-digit ZIP Codes on July 1, 1963, throughout the country. On October 1, 1963, Publication 59 was issued by the US Department of the Post Office, featuring abbreviations that are used with ZIP Code. It is a list of two letters of a state abbreviation written in capital letters. However, it had earlier been suggested that capitalization of abbreviations would be between two and five letters. The idea of a two-letter standard on ‘Publication 59’ was founded on a maximal 23 position line since it was universally accepted to be a more efficient line capacity basis for primary addressing systems. The two-letter abbreviation would then be followed by a lengthy city name which is also integrated with a multi-letter state, for example,’ Sacramento, Calif’ would be combined with the area’s ZIP Code. However, these abbreviations have not changed except in 1969, when the Canadian postal administration requested that Nebraska’s abbreviation is changed to NE from NB so that people can stop confusing it with the Canadian province of New Brunswick.

In 1944, Robert Moon, who is regarded as the inventor of the ZIP Code, submitted the idea while he worked as a postal inspector. He is attributed with the first three digits which describe the sec center also known as the sectional center facility (SCF). The SCF is the one that sorts the mail with the first three digits in their ZIP Codes to all post offices before being sorted by the final two digits and later sent to corresponding offices. However, sectional centers are not accessible to the public and do not deliver mail, with the majority of the employees working the night shift. In the afternoon, the mail collected at post offices are then sent to the specified SCF and sorted during the night. The last two digits in the bigger cities corresponded with the older postal zone number. However, in 1967, it became a must for both third and second-class bulk mailers thus leading to the adoption of the system.


in 1983, the US Postal Service expanded the ZIP Code system to ZIP+4, also known 'plus four codes', 'add-ons' or 'add-on codes'. The system uses a simple five-digit code along with an extra four digits that identify a geographical segment within a five-digit delivery area, such as a group of apartments or a city block. The use of an extra identifier is to help in the delivery and sorting of mail. The rule of Post Office Boxes requires that every box contains its ZIP plus four codes together with an add-on code which is usually the box number’s last four digits, zero plus the last three digits of the box number. Lastly, if the box’s digits are less than four, zeros are added before the box number thus forming a four-digit number.

Postal Barcode

A ZIP Code is usually interpreted to an Intelligent Mail barcode which is printed on the mail piece, thus making simpler for the automated machines to categorize mail. The sender can print a barcode, but it is recommended that you allow the post office to do it when they are processing the piece. The post office uses OCR technology, although sometimes a human being may be required to read and enter the address. Upon postage, clients who send bulk mail are usually given a discount in case they presort the mail or print the barcode. Mail lists need to be standardized with updated versions of the Coding Accuracy Support System (CASS) an accredited software which not only verifies but also adds correct ZIP plus four Code as well as an extra two digits which represent the specific delivery point. The mail is supposed to be sorted in a particular scheme, an 11-digit zip with at least 150 mailpieces, and accompanied by documentation of verification. PAVE-certified software takes such steps, including the printing the address labels for the barcode as well as the tray or sack tags. Therefore, every one of the mailable points in the states comprises of a 12-digit number.


In the US, delivery points are designated by ZIP Codes a well as stations overseas including territories for its armed forces. The Republic of Marshall Islands, the Republic of Palau and the independent nations of the Federated States of Micronesia each have integrated ZIP Codes in the US postal system which is operated by a Compact of Free Association. Similarly, mail that is sent overseas to any US diplomatic missions is often handled as though it was intended for an address in Washington, DC. Diplomatic mail comprises of a four-digit diplomatic mail pouch number, which is not only used as a building number but also the city where the embassy is located, and is combined with the term place, forming the name of the street.

ZIP Codes are usually labeled with numbers starting with the first digit which represents a specific group of US states. Both the second and third digits represent an area in that group, the alphabetical order then comes after the numerical order. Although the idea of ZIP Codes was meant for systematic postal delivery, a few circumstances exist where ZIP Codes might cross state boundaries, a good example being military facilities.


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