zip codes

The ZIP code is the system of postal codes used by the United States Postal Service (USPS). The name ZIP, an acronym for Zone Improvement Plan[1] that is properly written in capital letters, was chosen to suggest that the mail travels more efficiently, and therefore more quickly, when senders use it. The basic 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

Wikimedia Foundation Inc.

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 submitted his proposal in 1944 while working as a postal inspector.

The post office only gives credit to Moon for the first three digits of the ZIP code, which describe the region of the country. In most cases, the last two digits coincide with the older postal zone number, thus:

John Smith

3256 Epiphenomenal Avenue

Minneapolis, Minnesota 55416

Wikimedia Foundation Inc.

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. Curiously, the only time the Postal Service issued a stamp promoting the ZIP code, in 1974, Mr. ZIP was not depicted.

ZIP+4

In 1983, the U.S. Postal Service began using an expanded ZIP-code system called "ZIP+4", often called "plus-four codes" or "add-on codes."

Wikimedia Foundation Inc.

200 2nd Ave. South #358

St. Petersburg, FL 33701-4313

Furthermore, recently the Postal Service started a "Find a ZIP Code" feature on its website, which provides an address format that is most compatible with its optical character recognition, or OCR, scanners:

WIKIMEDIA FOUNDATION INC

200 2ND AVE S # 358

SAINT PETERSBURG FL 33701-4313

A ZIP+4 code uses the basic five-digit code plus an additional four digits to identify a geographic segment within the five-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 a Postnet barcode on the face of the mailpiece that corresponds to 11 digits. 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, if the box number consists of fewer than four digits, enough zeros added to the front of the box number to make it a four-digit 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 to make it easier for automated machines to sort. 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 it processes the piece. 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; postcard printers have begun blocking a section off where the barcode will go to compensate.)

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 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[2]. The last digit is always a check digit, which is obtained by summing the No. 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, 3%10=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 to bypass the MOCR and go straight to a barcode scanning machine. For that reason, even though courtesy reply mail and metered reply mail are mailed with a stamp or a postage-meter imprint, they typically carry a FIM code, namely FIM A, to indicate that the Postnet bar code is present. The FIM D barcode is used for computer-generated indicia from online postage meters.

Structure and allocation

By geography

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; afterward, the numerical order often follows the alphabetical order.

Generally, the first three digits designate a sectional center facility, the mail-sorting and -distribution center for an area. A sectional center facility may have more than one three-digit code assigned to it. For example, the Northern Virginia sectional center facility in Merrifield is assigned codes 220, 221, 222 and 223.

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 U.S. Virgin Islands and APO/FPO military addresses for personnel stationed in Europe. The lowest ZIP code is in Holtsville, New York (00501, a unique ZIP Code for the U.S. Internal Revenue Service center there). Other low ZIP codes are 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), 10036 (New York City), 19103 (Philadelphia), 20008 (Washington, D.C.), 30303 (Atlanta) and 33130 (Miami). (Each of those cities has many other ZIP codes besides the example shown.) From there, the numbers increase heading westward and northward. For example, 40202 is in Louisville, 50309 in Des Moines, Iowa, 60601 in Chicago, 75201 in Dallas, 80202 in Denver, 94111 in San Francisco, 98101 in Seattle, and 99950 in Ketchikan, Alaska.

The 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 (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.

For example, U.S. government agencies in and around the nation's capital are assigned ZIP codes starting with 20200 to 20599, which are Washington, D.C., ZIP codes, even if they are not located in Washington itself. While the White House itself is located in ZIP code 20006, it has the ZIP code 20500. The Nuclear Regulatory Commission is located in Rockville, Maryland, at ZIP code 20852, but has been assigned by the Postal Service the address "Washington, DC 20555". The United States Patent and Trademark Office used to be located in Crystal City, Virginia at ZIP Code 22202 but was assigned by the Postal Service the address "Washington, DC 20231"; however, since its move to Alexandria, Virginia, it uses the ZIP+4 code 22313-1450.

Rarely, a locality is assigned a ZIP code that does not match the rest of the state. This is when the locality is so isolated that it is served from a sectional center in another state. For example, Fishers Island, New York, bears the ZIP code 06390 and is served from Connecticut — all other New York ZIP codes (excepting those at Holtsville for the IRS) begin with "1".

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.

Default place names are typically 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 among seven ZIP codes assigned to "Aurora", "Englewood" or "Littleton" as its "default" place names. Thus, postally speaking, the city of Centennial and its 100,000 residents do not exist - they are part of Aurora, Englewood or Littleton. In the ZIP-code directory, Centennial addresses are listed under those 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 Web site 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 that 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 that 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.

"Acceptable place names" also come into play in areas of the country where many citizens identify more strongly with a particular urban center than the municipality they actually live in. For example, Allegheny County, Pennsylvania has 130 distinct municipalities, but many of the county's residents, and even some residents of adjacent counties, commonly use "Pittsburgh, PA" as their postal address.

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, New Hampshire, 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. Another example is ZIP code 08043, which corresponds to the census-designated place of Kirkwood, NJ but actually serves the entirety of Voorhees Township, NJ.

The postal designations for place names become de facto locations for their addresses, and as a result it is 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 codes. 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 cities can be the "default" place name for addresses within the ZIP code.

This confusion also can have financial implications for local governments, because mail volume is among the factors used by the US Census to estimate population changes between decennial census enumerations. Sometimes local officials in a community that is not the "default" place name for a zip code but is an "acceptable" place name will advise residents to always use the name of the community, because if the census estimate of that town's population is low they will get less of various State and Federal funds that are computed based on population.

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 codes become effective once announced, and a grace period (e.g., one year) is provided in which the new and old 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 the 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 boundaries 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.

By type/use

There are three types of ZIP codes: unique (assigned to a single high-volume mailer), P.O.-box-only (used only for P.O. 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 ZIP codes, such as 81009 for the Federal Citizen Information Center of the U.S. General Services Administration (GSA) [3] in Pueblo, Colorado; 43210 for The Ohio State University in Columbus, Ohio; 92803 for Disneyland in California and 32830 for Walt Disney World in Florida; 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 the First Family to receive private mail [4]. An example of a P.O.-box-only ZIP code is 22313, which is used for P.O. 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, and also some PO boxes)
• 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)

Other uses

Delivery services other than the USPS, such as FedEx, United Parcel Service and DHL require a ZIP code for 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 has data that includes the latitude and longitude of the center-point of some ZIP codes (called ZIP Code Tabulation Areas or ZCTAs). The Census Bureau does not keep up-to-date datasets of all ZIP codes. Complete datasets are commercially available.

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 to collect corporate purchasing-pattern data. The corporation or specialists then analyze these data to determine the location of new business establishments. ZIP-coded data are also used in analyzing geographic factors in risk, an insurance-industry and banking practice pejoratively known as redlining. This can cause problems (e.g. expensive insurance) for people living near a town with a high crime rate and sharing its ZIP code, while they themselves actually live in a relatively crime free town (e.g. south west part of 94303).

Pop culture

• In 1964, at the height of his popularity, Smokey Bear, mascot of the U.S. Forest Service, received so much fan mail that he was assigned his own ZIP code, 20252.
• Black Rocky City, NV, a city formed entirely by the presence of the Burning Man Festival for about two weeks each summer, has been issued its own ZIP code: 89412
• ZIP code 12345 is a unique code that belongs to a General Electric factory in Schenectady, New York. Each year, the facility is bombarded with letters from children to Santa Claus, addressed to "North Pole 12345".
• Game-show viewers of the 1960s–1980s became familiar with announcer spots for the Spiegel catalog and the company's address, "Chicago 60609."
• In a popular episode of the 1960s TV series Batman, the villain Chandell mentioned being located in ZIP code 9999979. Although the producers knew this could not be an actual ZIP code, it is likely that this was used as an attempt to bring more attention to ZIP codes as part of mailing addresses.
• In the movie based on the Green Acres television series, Hooterville's ZIP code was given by Mr. Drucker as "40516 and a half". 40516 is in Lexington, Kentucky.
• Public-service announcements for the government's Federal Citizen Information Center's Consumer Information Catalog have become famous for its ZIP code in Pueblo, Colorado, 81009 (which is assigned uniquely to the Center).
• Newton Falls, Ohio, has the ZIP code 44444. During the 1970s, signs at the municipal boundaries proclaimed "Newton Falls has zip!" The slogan is now in use on a Web site, 44444.com.
• ZIP codes can take on a certain amount of cachet or become bywords: 90210, in Beverly Hills, California, probably the most famous example, appears in the titles of two Beverly Hills–centered television shows: Beverly Hills 90210 and Dr. 90210.
• On Seinfeld, Newman, a USPS employee, tells his girlfriend "I'll tell you a little secret about ZIP codes. They're meaningless!"
• The PBS children's series ZOOM made frequent musical use of 02134, the ZIP code of WGBH, the show's originating station in Boston, Massachusetts.
• In the "The Day the Violence Died" episode of The Simpsons, the formerly bankrupt Meyers Studios (creators of the Itchy & Scratchy show) are revived after the owner of the defunct studio successfully sues the U.S. Postal Service for copyright infringement on a character that resembles Mr. ZIP, aptly named Manic Mailman.
• In another Simpsons episode, "Sunday, Cruddy Sunday," Lisa Simpson asks a postal employee, during a class trip to the post office, what the purpose of the ZIP+4 code is. Little does she know that she is being monitored by authorities who suspect she has discovered that the code has no importance; the tour guide, Postmaster Bill, mentions under his breath that they are "citizen-relocation codes" and "hopefully we'll never need them".
• Starting in 2000, National Geographic magazine instituted a regular monthly feature to focus on the community in one ZIP code.
• Mugs and T-shirts for sale in Cambridge, Massachusetts, name 02138, the Harvard Square (which includes Harvard University) ZIP code.
• A new magazine featuring Harvard graduates is entitled 02138.
• In the current television mystery series Veronica Mars, "09ers" is a slang term used to describe students whose families live in the fictitious ZIP code 90909, the affluent part of Neptune, California.
• The comic strip "Peanuts" featured a character called 5, whose last name is a ZIP code in Sebastopol, California: 95472.
• The television show Full House makes fun of Wayne, New Jersey's palindromic zip code (07470).