ZIP Code is the postal code used by the United States Postal Service, 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 plus four more digits which allow a piece of mail to be even more accurately directed to a very small geography. ZIP Code was originally registered as a trademark by the U.S. Postal Service, but its registration has since expired.
The term "ZIP Code" is also used in the Philippines to name its postal codes. The Philippine ZIP Code is used by the Philippine Postal Corporation. Unlike American ZIP Codes, Philippine ZIP Codes are four-digit numbers without any extensions. While the cities of Metro Manila use more than one code, towns and cities outside Metro Manila are assigned only one code for every town and city.
The postal service implemented postal zones for large cities in 1943. For example:
3256 Epiphenomenal Avenue
Minneapolis 16, Minnesota
The "16" 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:
3256 Epiphenomenal Avenue
Minneapolis, Minnesota 55416
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.
In addition, two-letter abbreviations were introduced for states, eliminating the need to write the state's name out in full. For example, California is CA. Abbreviations are also assigned for U.S. territories 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.
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). This may also be used for mail to many US diplomatic missions overseas.
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." 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, 9999 for general delivery, and other high-numbered add-on codes for business reply mail and requests for special cancellation of stamps. 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, 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 presort in 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). These two digits are usually the last two of the street address or box number, though non-numeric points with names or letters are assigned DP numbers by the local post office. However, when house numbers differ only by a letter suffix, e.g., 120 and 120A, the delivery point may be the same. 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 fluorecent 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.
Structure and allocation
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.
Geographically, many of the lowest ZIP Codes are in the New England region, since these begin with '0'. Also in the '0' region are Puerto Rico, the US Virgin Islands and APO/FPO military addresses for personnel stationed in Europe. Some low zip codes are: 00501 for Holtsville, New York (a unique ZIP code for the U.S. Internal Revenue Service center there); 00601 for Adjuntas, Puerto Rico; 01001 for Agawam, Massachusetts, and 01002 for Amherst, Massachusetts.
The numbers increase southward along the East Coast, such as 02115 (Boston, Massachusetts), 10036 (New York City), 19103 (Philadelphia, Pennsylvania), 20008 (Washington, DC), 30303 (Atlanta, Georgia), and 33130 (Miami, Florida). From there, the numbers increase 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.
Map of ZIP code zones.
3-digit lists: 0-1 • 2-3 • 4-5 • 6-7 • 8-9The first digit of the ZIP code is allocated as follows:
0 = Connecticut (CT), Massachusetts (MA), Maine (ME), New Hampshire (NH), New Jersey (NJ), Puerto Rico (PR), Rhode Island (RI), Vermont (VT), Virgin Islands (VI), APO Europe (AE), FPO Europe (AE).
1 = Delaware (DE), New York (NY), Pennsylvania (PA)
2 = District of Columbia (DC), Maryland (MD), North Carolina (NC), South Carolina (SC), Virginia (VA), West Virginia (WV)
3 = Alabama (AL), Florida (FL), Georgia (GA), Mississippi (MS), Tennessee (TN), APO Americas (AA), FPO Americas (AA).
4 = Indiana (IN), Kentucky (KY), Michigan (MI), Ohio (OH)
5 = Iowa (IA), Minnesota (MN), Montana (MT), North Dakota (ND), South Dakota (SD), Wisconsin (WI)
6 = Illinois (IL), Kansas (KS), Missouri (MO), Nebraska (NE)
7 = Arkansas (AR), Louisiana (LA), Oklahoma (OK), Texas (TX)
8 = Arizona (AZ), Colorado (CO), Idaho (ID), New Mexico (NM), Nevada (NV), Utah (UT), Wyoming (WY)
9 = Alaska (AK), American Samoa (AS), California (CA), Guam (GU), Hawaii (HI), Northern Mariana Islands (MP), Oregon (OR), Washington (WA), APO Pacific (AP), FPO Pacific (AP).
Other U.S. territories have codes starting with 9. However, with the expansion of ZIP codes, the assignment of the first digit to a group of states has broken down. For example, ZIP codes beginning with 0 and 1 are in use in New York; beginning with 2 and 5, in the District of Columbia; and beginning with 7 and 8, in Texas.
The next two digits represent the sectional center facility (sorting facility for a region) (e.g. 432xx = Columbus OH), and the fourth and fifth digits represents the area of the city (if in a metropolitan area), or a village/town (outside metro areas): 43209 (4=Ohio,32=Columbus,09=Bexley). When a sectional center facility's area crosses state lines, that facility is assigned separate three-digit prefixes for the states that it serves; thus, it is possible to identify the state associated with any ZIP code just by looking at the first three digits. Often, the last two digits are assigned in alphabetical order to each community for sortation centers that serve multiple cities.
It is important to note that despite the geographic derivation of most ZIP Codes, the codes themselves are not geographic regions, but simply categories for grouping mailing addresses. ZIP Code "areas" can overlap, be subsets of each other, or be artificial constructs with no geographic area. Similarly, in areas without regular postal routes (rural route areas) or no mail delivery (undeveloped areas), ZIP Codes are not assigned or are based on sparse delivery routes, and hence the boundary between ZIP Code areas is undefined.
ZIP codes only loosely tied to cities
An address's ZIP code and the "city" name written on the same line do not necessarily mean that that address is within that city. The Postal Service designates a single "default" place name for each ZIP code. This may be an actual incorporated town or city, a subentity of a town or city, or an unincorporated census-designated place. Additional place names, also of any of these types, may be recognized as "acceptable" for a certain ZIP code. Still others are deemed "not acceptable", and if used, may result in a delay in mail delivery.
The ideal, of course, would be that the "default" place name is the actual city or town that the address is located in. However, for many cities that have incorporated since ZIP codes were introduced, the actual city name is only "acceptable" and not the "default" place name. Many databases automatically assign the "default" place name for a ZIP code, without regard to any "acceptable" place names. For example, Centennial, Colorado, the largest city to incorporate in U.S. history, is divided between seven ZIP codes, each of which either has "Aurora", "Englewood" or "Littleton" as its "default" place name. Thus, postally speaking, the city of Centennial and its 100,000 residents do not exist - they are postally part of Aurora, Englewood or Littleton: in the ZIP code directory, Centennial addresses are listed under these three cities. And since it is "acceptable" to write "Centennial" in conjunction with any of the seven ZIP codes, one can write "Centennial" in an address that is actually in Aurora, Englewood, or Littleton, as long as it is in one of the shared ZIP codes.
"Acceptable" place names are often added to a ZIP code in cases where the ZIP code boundaries divide them between two or more cities, as in the case of Centennial. However, in many cases only the "default" name can be used, even when many addresses in the ZIP code are in another city. For example, approximately 85% of the area served by the ZIP code 85254, to which the place name "Scottsdale, Arizona" is assigned, is actually inside the city limits of neighboring Phoenix. This is because the post office that serves this area is in Scottsdale. This has led some residents of the ZIP code to believe that they live in Scottsdale when they actually live in Phoenix. A city of Scottsdale website listing the positive and negative aspects of the city mentioned the 85254 ZIP code as a positive aspect, because "Scottsdale" is being used for businesses located outside the city limits in Phoenix. This phenomenon is repeated across the country. The previously mentioned Englewood is a land-locked, inner ring suburb that was built out by the 1960s. Its post office served the area which is now the high-growth southern tier of the Denver metropolitan area, and ZIP codes in this area were assigned "Englewood" as their "default" place name. An employment center as large as downtown Denver has grown in this area, and its office parks are the headquarters for many internationally-recognized corporations. Even though they are actually located in other cities, they indicate "Englewood" as their location, as this is the "default" postal place name. As a result, there are really two "Englewoods" - the actual city, small and with a largely working-class residential population; and a number of miles away, the postal "Englewood", a vast suburban area of upscale subdivisions and office parks which have nothing to do with the City of Englewood yet share a split identity with it solely because of ZIP codes. People who say that they live or work in "Englewood" and identify closely with it may rarely enter the actual city of that name.
Finally, many ZIP codes are for villages, census-designated places, portions of cities, or other entities that are not municipalities. For example, ZIP code 03750 is for Etna, NH, but Etna is not a city or town; it is actually a village district in the town of Hanover, which itself is assigned the ZIP Code 03755.
The postal designations for place names become de facto locations for their addresses, and as a result, it is very difficult to convince residents and businesses that they actually are located in another city or town different from the "default" place name associated with their ZIP code. Because of the confusion and lack of identity generated by this situation, some cities, such as Signal Hill, California, have successfully petitioned the Postal Service to change ZIP code boundaries or create new ZIP codes, so that their city can be the "default" place name for addresses within the ZIP code.
Division and reallocation of ZIP codes
Like area codes, ZIP Codes are sometimes divided and changed, especially when a rural area becomes suburban. Typically, the new ZIP codes become effective once announced, and a grace period (e.g., one year) is provided in which the new and old ZIP codes are used concurrently, so that postal patrons in the affected area can notify correspondents, order new stationery, etc.
Most significantly, in rapidly developing suburbs, it is sometimes necessary to open a new sectional center facility, which must then be allocated its own three-digit ZIP-code prefix or prefixes. Such allocation can be done in various ways. For example, when a new sectional center facility was opened at Dulles Airport in Virginia, the prefix 201 was allocated to that facility; therefore, for all post offices to be served by that sectional center facility, the ZIP code changed from an old code beginning with 220 or 221 to a new code or codes beginning with 201. However, when a new sectional center facility was opened to serve Montgomery County, Maryland, no new prefix was assigned. Instead, ZIP codes in the 207 and 208 ranges, which had previously been assigned alphabetically, were reshuffled so that 207xx ZIP codes in Montgomery County were changed to 208xx codes, while 208xx codes outside that county were changed to 207xx codes. Because Silver Spring (whose postal area includes Wheaton) has its own prefix, 209, there was no need to apply the reshuffling to Silver Spring; instead, all mail going to 209xx ZIP codes was simply rerouted to the new sectional center facility.
ZIP codes also change when postal boundaries are realigned. For example, at the same time at which the above-noted change in Montgomery County took place, and under pressure from then D.C. Mayor Marion Barry, the USPS realigned the postal boundary between the District of Columbia and Maryland to match the actual boundary. Previously, many inner suburbs, such as Bethesda and Takoma Park, had been in the Washington, D.C., postal area. As a result of the change, ZIP codes in Maryland beginning with 200 were changed to new ZIP codes beginning with 207, 208, or 209, depending on their location, and ZIP codes straddling the D.C.-Maryland line were split. For example, 20014 (Bethesda) became 20814, while the Maryland portion of 20012 (Takoma Park) became 20912.
There are three types of ZIP codes: unique (assigned to a single high-volume mailer), PO box-only (used only for PO boxes at a given facility, not for any other type of delivery), and standard (all other ZIP codes). As examples of unique ZIP codes, certain governmental agencies, universities, businesses, or buildings that receive extremely high volumes of mail have their own unique ZIP Code, such as 81009 for the Federal Citizen Information Center (FCIC) of the U.S. General Services Administration (GSA)  in Pueblo, Colorado, 21250 for University of Maryland Baltimore County, 30385 for BellSouth in Atlanta, 12345 for General Electric in Schenectady, New York, 10048 for the World Trade Center complex in New York, New York (until its destruction on September 11, 2001) and 77230 for victims of Hurricane Katrina being housed at the Houston Astrodome. The White House has its own secret ZIP+4 Code, separate from the publicly-known 20500, for the President of the United States and his family to receive private mail . An example of a "PO box-only" ZIP code is 22313, which is used for PO boxes at the main post office in Alexandria, Virginia. In the area surrounding that post office, home and business mail delivery addresses use ZIP code 22314, which is thus a standard ZIP code.
The above will be made clearer by examining the allocation of ZIP codes in Princeton, New Jersey:
08540 - standard (deliveries in most of the Princeton postal area)
08541 - unique (Educational Testing Service)
08542 - standard (deliveries in the central area of the borough of Princeton)
08543 - PO box only (PO boxes at the main post office)
08544 - unique (Princeton University)
Another type: M - Military 34036, Military - Armed forces Americas (except Canada)
Delivery services other than the USPS, such as Federal Express, United Parcel Service, and DHL require a ZIP code for the optimal internal routing of a package. This spares customers from being required to use some other routing designator, such as the IATA code of the destination airport or railhead.
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. Point of Sale cashiers sometimes ask consumers what ZIP Code they live in in order to collect corporate purchasing pattern data. The corporation or specialists then analyze these data to determine the location of new business establishments. Finally, ZIP-coded data is also used in analyzing geographic factors in risk, an insurance industry and banking practice pejoratively known as redlining.