A ZIP code is the postal code used by the United States Postal Service. ZIP is an acronym for the Zone Improvement Plan, but was also cleverly meant to imply the fact that mail travels more efficiently (and therefore faster) when senders use it. The basic ZIP code consists of five numerical digits. An extended ZIP+4 code includes the five digits of the ZIP code plus four digits which allow a piece of mail to be delivered to a specific address.
The postal service implemented postal zones for large cities in 1943. For example:
- John Smith
- 3256 Epiphenomenal Avenue
- Minneapolis 16, Minnesota
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. In most cases, the last two digits of the ZIP code coincide with the postal zone number, thus:
- John Smith
- 3256 Epiphenomenal Avenue
- Minneapolis, Minnesota 55416
In 1983, the Postal Service began using an expanded ZIP code system called "ZIP+4," which are often called "plus-four codes." A ZIP+4 code uses of 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 or a group of apartments or 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, but it helps the Postal Service direct mail more efficiently and accurately because it reduces handling and significantly decreases the potential for human error and possibility of misdelivery.
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, or the post office will put one on when they receive it. If the post office does it, they either have a machine OCR it, or have a human 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 only a simple and often free font, and the knowledge of at least the main 5-digit code, if all 9 are not available. An additional two digits are usually used to indicate the exact delivery point, so that every single mailable point in the country has its own 11-digit number. These two digits are usually the last two of the street address or box number, though non-numeric points with names or letterss are assigned DP numbers by the local post office. The last digit is always a check digit, which is obtained by adding up the 5-, 9-, or 11-digits, then subtracting the last digit of that result from 10. (Thus, the check digit for 10001-0001 00 would be 7, or 1+1+1=3 and 10-3=7.) The sender needs only to type something like /100010001007/ in the 12-point POSTNET font to create the code for printing.
ZIP codes are used not only for tracking of mail, but in gathering geographical statistics in the United States. The U.S. Census Bureau keeps track of the latitude and longitude of the center-point of each ZIP code, a database which numerous other companies sell. The data are often used in direct mail campaigns in a process called ZIP code marketing, developed by Martin Baier. ZIP-coded data is also used in analyzing geographic factors in risk, an insurance industry and banking practice pejoratively known as redlining.
Robert Moon, an employee of the post office, is considered the father of the ZIP code, which was adopted in 1963. 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.
ZIP codes are numbered with the first digit representing a certain group of U.S. states, the second and third digits together representing a region in that group (or perhaps a large city), and the fourth and fifth digits representing more specific areas, such as small towns or regions of that city. The main town in a region (if applicable) often gets the first ZIP codes for that region; afterwards, the numerical order often follows the alphabetical order. Like area codes, ZIP codes are sometimes divided and changed, expecially when a rural area becomes suburban.
Geographically, the lowest ZIP codes are in the New England region such as 02107 in Boston, Massachusetts. The numbers increase southward along the East Coast, such as 10036 (New York City), 20500 (Washington, DC), 30303 (Atlanta, Georgia). From there, the numbers begin increasing heading westward and northward. For example, 40202 is in Louisville, Kentucky, 50309 in Des Moines, Iowa, 60601 in Chicago, Illinois, 75201 in Dallas, Texas, 80202 in Denver, Colorado, 94111 in San Francisco, California, 98101 in Seattle, Washington, and 99950 in Ketchikan, Alaska.
Some places with extremely large volumes of mail have their own full ZIP code, including 81009 for the Federal Citizen Information Center (FCIC) of the U.S. General Services Administration (GSA)  in Pueblo, Colorado, and 10048 for the World Trade Center complex in New York, New York (at least until September 11, 2001, when the complex was brought to utter ruin). The White House has its own secret ZIP code, separate from the regularly-occurring ZIP code, for the U.S. President and his family to receive private mail.
The United States Post Office used a cartoon character, Mr. ZIP, to promote use of the ZIP code.