A ZIP Code is the postal code used by the United States Postal Service (USPS), which always writes ZIP with capital letters. ZIP is an acronym for the Zone Improvement Plan, but was also meant to suggest that mail travels more efficiently (and therefore faster) when senders use it. The basic ZIP Code format consists of five numerical digits. An extended ZIP+4 code includes the five digits of the ZIP Code, a hyphen, and then four more digits, which allow a piece of mail to be directed to a more precise location than by the ZIP code alone. ZIP Code was originally registered as a trademark by the U.S. Postal Service, but its registration has since expired.

Background

The postal service implemented postal zones for large cities in 1943. For example:

John Smith

3256 Epiphenomenal Avenue

Minneapolis 16, Minnesota

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200 2nd Ave. South #358

St. Petersburg 1, Florida

The "16" in the first example and "1" in the second is the number of the postal zone within the city.

By the early 1960s 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 most cases, the last two digits of the ZIP Code coincide with the older postal zone number, thus:

John Smith

3256 Epiphenomenal Avenue

Minneapolis, Minnesota 55416

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200 2nd Ave. South #358

St. Petersburg, Florida 33701

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. He was often depicted with a legend such as "USE ZIP CODES" in the selvage of panes of stamps, or on labels contained in, or the covers of, booklet panes of stamps. Ironically, the only time the Postal Service issued a stamp promoting the ZIP Code, in 1974, Mr. Zip was not depicted.

Postal abbreviations

In addition, two-letter abbreviations were introduced for states, eliminating the need to write the state's name out in full.

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200 2nd Ave. South #358

St. Petersburg, FL 33701

For example, California is CA. Abbreviations are also assigned for U.S. insular areas like Puerto Rico (PR) and American Samoa (AS), as well as for several former U.S. Trust Territories in the Pacific, such as the Federated States of Micronesia (FM), which are now separate countries. Nebraska was originally assigned the abbreviation (NB) but that was changed to (NE) to avoid confusion with the Canadian province of New Brunswick

Similarly, US military addresses also have their own abbreviations. Mail to these addresses is sent to the Army (or Air Force) Post Office (APO) or Fleet Post Office (FPO). Instead of a state abbreviation, (AA), (AE) or (AP) is used. These may also be used for mail to many US diplomatic missions overseas.

For more efficient use of its optical-character-recognition mail-sorting equipment, the Postal Service advises mailers to eliminate punctuation from addresses on mail pieces: for example, "St. Petersburg, FL" should be rendered "Saint Petersburg FL".

ZIP+4

In 1983, the US Postal Service began using an expanded ZIP Code system called "ZIP+4", which are often called "plus-four codes" or "add-on codes."

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200 2nd Ave. South #358

St. Petersburg, FL 33701-4313

Furthermore, recently the US Postal Service started a "Find a ZIP Code" feature on its Web site, which provides an address format that is most compatable with its OCR scanners:

WIKIMEDIA FOUNDATION INC.

200 2ND AVE S

358

SAINT PETERSBURG FL 33701-4313

A ZIP+4 code uses the basic 5-digit ZIP plus an additional 4-digits to identify a geographic segment within the 5-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail, or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery. Use of the plus-four code is not required except for certain presorted mailings. In general, mail is read by a Multiline Optical Character Reader (MLOCR) that instantly determines the correct ZIP+4 code from the address and?along with the even more specific Delivery point?sprays an 11-digit POSTNET barcode on face of the mailpiece. This technology has greatly increased the speed and accuracy of mail delivery and in turn, kept costs nearly constant for over a decade.

For Post Office boxes, the general (but not invariable) rule is that each box has its own ZIP+4 code. The add-on code is often either the last four digits of the box number or 0 plus the last three digits of the box number. However, there is no uniform rule, so the ZIP+4 code must be looked up individually for each box.

It is common to use add-on code 9998 for mail addressed to the postmaster (to which requests for pictorial cancellations are usually addressed), 9999 for general delivery, and other high-numbered add-on codes for business reply mail. For a unique ZIP code (explained below), the add-on code is typically 0001.

Postal bar code

The ZIP Code is often translated into a barcode called POSTNET, that is printed on the mailpiece as well, to make it easier for automated machines to sort the mail. Unlike most barcode symbologies, POSTNET uses long and short bars, not thin and thick bars. The barcode can be printed by the person who sends the mail (some word processing programs such as Word Perfect and Microsoft Word include the feature), or the post office will put one on when they receive it. The post office generally uses OCR technology, though a human may have to read the address if absolutely necessary. (The automated machinery has the unfortunate tendency to paste the coding over the bottom half-inch of postcards, often obliterating the signature.)

People who send bulk mail can get a discount on postage if they have pre-printed the barcode themselves. This requires more than just a simple font; mailing lists must be standardized with up-to-date CASS certified software that adds/verifies a full, correct ZIP+4 code and an additional two digits representing the exact Delivery point. Furthermore, mail must be presorted in a specific scheme and be accompanied by documentation verifying this. These steps are usually done with PAVE certified software that also prints the barcoded address labels and barcoded sack or tray tags.

This means that that every single mailable point in the country has its own 11-digit number (at least in theory). The Delivery point digits (the 10th and 11th digits) are calculated based on the primary or secondary number of the address. The USPS publishes the rules for calculating the Delivery point in a document called the CASS Technical Guide. The last digit is always a check digit, which is obtained by summing the 5, 9, or 11 digits, taking the Modulo base 10 of this sum (i.e. the remainder after dividing by 10,) and finally subtracting this from 10. (Thus, the check digit for 10001-0001 00 would be 7, or 1+1+1=3 and 10−3=7.) An application needs only to print something like /100010001007/ in the 12-point POSTNET font to create a valid barcode. The slashes "/" are translated into start/stop characters (one long bar,) and each digit is translated into a sequence of two long bars and three short bars.

On business reply mail, the FIM code primarily indicates the orientation (facing) of the mailpiece, since there is generally not a stamp or postage meter imprint containing fluorescent ink (which is usually used by the facing machine to orient mail.) Additionally, FIM codes A and C indicate that a POSTNET bar code is present, allowing this mail bypass the Multiline Optical Character Reader and go straight to a Barcode Scanning Machine (BCS). For that reason, even though courtesy reply mail and metered reply mail are mailed with a stamp or postage meter imprint, they typically carry a FIM code, namely FIM A, to indicate that the POSTNET bar code is present.