ZIP codes are a system of postal codes used by the United States Postal Service (USPS) since 1963. The term ZIP, an acronym for Zone Improvement Plan,^[1] is properly written in capital letters and was chosen to suggest that the mail travels more efficiently, and therefore more quickly, when senders use the code in the postal address. The basic format consists of five decimal numerical digits. An extended ZIP+4 code, introduced in the 1980s, includes the five digits of the ZIP code, a hyphen, and four more digits that determine a more precise location than the ZIP code alone. The term ZIP code was originally registered as a servicemark (a type of trademark) by the U.S. Postal Service, but its registration has since expired.^[2]

ZIP codes designate only delivery points within the United States and its dependencies, as well as locations of its armed forces. There are no ZIP codes reserved for designating mail bound for foreign destinations (with the exception of U.S. military units stationed outside of the United States), and therefore, international outbound mail should not include a ZIP code in the delivery address. The last line of a foreign address must only show the name of the country of destination.^[3]

Background[link]

A 1963 U.S. Post Office sign encouraging the use of ZIP codes

"Use Zip code" labels were also used to promote the use of a ZIP code.

The United States Post Office Department (USPOD) implemented postal zones for large cities in 1943. For example:

Mr. John Smith

3256 Epiphenomenal Avenue

Minneapolis 16, Minnesota.

The "16" was 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. Simultaneously with the introduction of the ZIP code, two-letter state abbreviations were introduced. These were to be written with both letters capitalized. The reason for the two-letter abbreviations is that it was thought that a long city name coupled with a multi-letter state abbreviation (e.g. Mass. for Massachusetts; Ca., Cal., or Calif. for California; Pa., Penn., or Penna. for Pennsylvania) would be too long for address labels used on magazines when the ZIP code was added. 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.^[4] The post office gives credit to Moon for only the first three digits of the ZIP code, which describe the sectional center facility (SCF) or "sec center." An SCF is a central mail processing facility with those three digits. The SCF sorts mail to all post offices with those first three digits in their ZIP codes. The mail is sorted according to the final two digits of the ZIP code and sent to the corresponding post offices in the early morning. Sectional centers do not deliver mail and are not open to the public (though the building may include a post office open to the public), and most of the workers are employed to work night shift. Mail picked up at post offices is sent to their own SCF in the afternoon, where the mail is sorted overnight. In the cases of large cities, the last two digits coincided with the older postal zone number,^{[citation needed]} thus:

Mr. John Smith

3256 Epiphenomenal Avenue

Minneapolis, MN 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, whom it called Mr. ZIP, to promote use of the ZIP code. He was often depicted with a legend such as "USE ZIP CODE" in the selvage of panes of stamps or on labels contained in, or the covers of, booklet panes of stamps.

ZIP+4[link]

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

A ZIP+4 code uses the basic five-digit code plus four additional 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. But initial attempts to promote universal use of the new format met with public resistance,^{[citation needed]} and today the plus-four code is not required. In general, mail is read by a multiline optical character reader (MLOCR) that almost instantly determines the correct ZIP+4 code from the address and—along with the even more specific delivery point—sprays a Postnet bar code on the face of the mail piece that corresponds to 11 digits—nine for the ZIP+4 code and two for the delivery point.

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 one of the following: the last four digits of the box number (e.g., PO Box 58001, Washington DC 20037-8001), zero plus the last three digits of the box number (e.g., PO Box 12344, Chicago IL 60612-0344), or, if the box number consists of fewer than four digits, enough zeros are attached to the front of the box number to produce a four-digit number (e.g., PO Box 52, Ambler PA 19002-0052). 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.^{[citation needed]} For a unique ZIP code (explained below), the add-on code is typically 0001.

Postal bar code[link]

This is the address shown in the text with the Postnet bar code for the five-digit ZIP code 55416.

This is the address shown in the text with the Postnet bar code for the nine-digit ZIP+4 code 33701-4313.

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 WordPerfect^[5] and Microsoft Word^[6] include the feature), or the post office will put one on when it processes the piece. In general, the post office uses OCR technology, though a human may have to read the address if absolutely necessary.

People who send bulk mail can get a discount on postage if they have printed the barcode themselves. This requires more than just a simple font; mailing lists must be standardized with up-to-date Coding Accuracy Support System (CASS)-certified software that adds and verifies a full, correct ZIP+4 code and an additional two digits representing the exact delivery point.^{[citation needed]} Furthermore, mail must be sorted 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 12-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.^[7] However when confronted with two addresses like 18 and 18C often CASS will assign the same 12-digit number to two distinct mail delivery points. The last digit is always a check digit, which is obtained by summing all 5, 9 or 11 digits, taking the residue modulo 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, since 1+1+1=3, 3≡3(mod 10) 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, in general, not a stamp or postage meter imprint containing fluorescent ink (which is usually used by the facing machine to orient mail).^{[citation needed]} 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.^{[citation needed]} 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 an 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[link]

By type/use[link]

USA postage stamp, 1973: "It all depends on ZIP code".

There are four types of ZIP codes: Unique (assigned to a single high-volume address), P.O.-box-only (used only for P.O. boxes at a given facility, not for any other type of delivery), Military (used to route mail for the U.S. military) 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 20505 for the Central Intelligence Agency in Washington, D.C., 81009 for the Federal Citizen Information Center of the U.S. General Services Administration (GSA)^[8] in Pueblo, Colorado; 30385 for BellSouth in Atlanta; and 21412 for Bancroft Hall, the midshipman dormitory at the United States Naval Academy. 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, as well as for the United States Patent and Trademark Office. In the area surrounding that post office, home and business mail delivery addresses use ZIP code 22314, which is thus a Standard ZIP code.

By geography[link]

Primary State Prefixes[link]

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 a group of delivery addresses within that region. 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.^{[citation needed]} Because ZIP codes are intended for efficient postal delivery, there are unusual cases where a ZIP code crosses state boundaries, such as a military facility spanning multiple states or remote areas of one state most easily serviced from an adjacent state. For example ZIP code 42223 spans Christian KY and Montgomery TN, and ZIP code 97635 spans Lake OR and Modoc CA.

In general, 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. In some cases, a sectional center facility may serve an area in an adjacent state, usually due to the lack of an appropriate location for a center in that region. For example, 739 in Oklahoma is assigned to Liberal, Kansas; 865 in Arizona is assigned to Gallup, New Mexico; and 961 in California to Reno, Nevada.

In terms of geographic location, many of the lowest ZIP codes, which begin with '0', are in the New England region. Also in the '0' region are New Jersey (non-contiguous with the remainder of the '0' area), Puerto Rico, the U.S. Virgin Islands and APO/FPO military addresses for personnel stationed in Europe, Africa, Southwest Asia and aboard vessels based in the waters adjoining those lands; APO/FPO addresses are also used by U.S. diplomatic and consulary facilities. The lowest ZIP code is in Holtsville, New York (00501, a ZIP code exclusively 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. Until 2001, there were six ZIP codes lower than 00501 that were numbered from 00210 to 00215 (located in Portsmouth, New Hampshire) and were used by the Diversity Immigrant Visa program to receive applications from non-U.S. citizens.^{[citation needed]}

The numbers increase southward along the East Coast, such as 02115 (Boston), 10001 (New York City), 19103 (Philadelphia), 20008 (Washington, D.C.), 30303 (Atlanta) and 33130 (Miami) (these are only examples, as each of these cities contain several ZIP codes in the same range). From there, the numbers increase heading westward and northward east of the Mississippi River, southward west of the Mississippi River, and northward on the west coast. For example, 40202 is in Louisville, 50309 in Des Moines, 60601 in Chicago, 63101 in St. Louis, 77063 in Houston, 80202 in Denver, 94111 in San Francisco, 98101 in Seattle, and 99950 in Ketchikan, Alaska (the highest ZIP code).

ZIP code zones in the USA

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), Army Post Office Europe (AE), Fleet Post Office 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), Army Post Office Americas (AA), Fleet Post Office 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), Marshall Islands (MH), Federated States of Micronesia (FM), Northern Mariana Islands (MP), Oregon (OR), Palau (PW), Washington (WA), Army Post Office Pacific (AP), Fleet Post Office Pacific (AP)

Secondary Regional Prefixes (123xx) and Local ZIP Codes (12345)[link]

The next two digits represent the sectional center facility (e.g. 477xx = Vanderburgh County, Indiana), and the fourth and fifth digits represent the area of the city (if in a metropolitan area), or a village/town (outside metro areas): 47722 (4=Indiana, 77=Vanderburgh County, 22=University of Evansville area). When a sectional center facility's area crosses state lines, that facility is assigned separate three-digit prefixes for the states that it serves.

In some urban areas, like 462 for Marion County, Indiana, the three-digit prefix will often exist in one county, while, in rural and most suburban areas, the prefix will exist in multiple counties, for example, the neighboring 476 prefix is found in part or in entirely in six counties: Gibson, Pike, Posey, Spencer, Vanderburgh, and Warrick. In some cases, an urban county may have more than one prefix. This is the case with Allen (467, 468), Lake (464, 463), St. Joseph (465, 466), and Vanderburgh (476, 477) Counties. Cities like Chicago, Houston, Los Angeles and New York have multiple prefixes within their city limits. In some cases, these may be served from the same SCF, such as in San Diego County, CA, where the three-digit prefixes 919 and 920 are used for suburban and rural communities, and 921 for the City of San Diego itself, although all three are processed through the same SCF facility.^{[citation needed]}

Despite the geographic derivation of most ZIP codes, the codes themselves do not represent geographic regions; in general, they correspond to address groups or delivery routes. As a consequence, ZIP code "areas" can overlap, be subsets of each other, or be artificial constructs with no geographic area (such as 095 for mail to the Navy, which is not geographically fixed). In similar fashion, 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, some residents in or near Haubstadt, Indiana, which has the ZIP code 47639, have mailing addresses with 47648, the ZIP code for neighboring Fort Branch, Indiana, while others living in or near Fort Branch have addresses with 47639. Many rural counties have similar logistical inconsistencies caused by the aforementioned sparse delivery routes, often known as Rural Routes or by some other similar designation.

For example, almost all U.S. government agencies in and around the 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".

In similar manner, the ZIP code for the Metropolitan Washington Airports Authority, a federally-chartered independent authority, is 20001-6000, even though the physical address of the Authority's office, "1 Aviation Circle",^[9] is in Arlington, Virginia.^{[citation needed]}

One current exception to this rule is the United States Patent and Trademark Office. When the USPTO was located in the Arlington, Virginia neighborhood of Crystal City, it was assigned by the Postal Service the address "Washington, DC 20231" despite being physically located in ZIP code 22202. However, the USPTO now uses a ZIP code (22313-1450 in ZIP+4) assigned to its current physical location in Alexandria, Virginia.^[10]

In rare circumstances, a locality is assigned a ZIP code that does not match the rest of the state; in other words, a ZIP code may cross state lines. In general, the locality is so isolated that it is most conveniently served from a sectional center in another state. Examples:

• Fishers Island, New York, bears the ZIP code 06390 and is served from Connecticut because the only ferry service is to Connecticut – all other New York ZIP codes (excepting those at Holtsville for the IRS) begin with "1".
• Some Texas ZIP codes are served from New Mexico (most notably some El Paso ZIP codes) and thus bear codes beginning with "885" (contiguous numerically with 870-884 NM) rather than "799".
• Returned government parcels from the District of Columbia are sent to ZIP codes beginning with "569", so that returned parcels are security checked at a remote facility (this was put into place after the 2001 anthrax attacks).
• Some Arkansas roads north of Bull Shoals Lake can best be accessed by the Protem, Missouri, delivery unit (ZIP code 65733).
• Fort Campbell (ZIP code 42223), primarily in Kentucky, main entrance is in Tennessee.
• Cincinnati/Northern Kentucky International Airport, despite being located across the Ohio River from Cincinnati in Hebron, Kentucky, uses ZIP code 45275 for mailing only, due to the fact the Kenton County Airport Board, which owns and operates the airport, maintains a P.O. Box physically located at the main post office in Cincinnati. The physical address of the airport is 2939 Terminal Drive, Hebron, Kentucky, 41048. In essence the mail for the Kenton County Airport Board has an Ohio ZIP code but the physical address of the airport is a Kentucky ZIP code.^[11]

ZIP codes and previous zoning lines[link]

A ZIP code's address and the city name written on the same line do not necessarily mean that address is within that city. The Postal Service designates one default place name for each ZIP code. This may be an actual incorporated town or city, a subentity of a large city or an unincorporated census-designated place. Additional place names 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 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 is divided among seven ZIP codes assigned to Aurora, Englewood or Littleton as its default place names. Thus, from the perspective of the U.S. Postal Service, the city of Centennial does not exist—it is 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 usually 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. People sometimes must use the name of a post office rather than their own 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 Scottsdale post office serves this area. This may lead some residents of the ZIP code to mistakenly believe they live in Scottsdale when they actually live in Phoenix.

Similarly, Missouri City, Texas, straddles Harris and Fort Bend counties. The portion within Harris County is within the ZIP code 77071, which must use the city name of Houston instead of Missouri City. At the same time, a small portion of the city of Houston is in Fort Bend County in the ZIP code 77489, and residents there must use the name Missouri City for their address even though they are in Houston.

This also occurs in some rural areas where portions of one town have their mail delivered by other post offices. For example, while most of the town of Plainfield, Massachusetts is in ZIP code 01070, some sections of town are in the ZIP code area for the neighboring town of Cummington with ZIP code 01026. Only the default name of Cummington is allowed in ZIP code 01026, so residents of parts of Plainfield must list their address as being in Cummington.

These phenomena are repeated across the country. The previously mentioned Englewood, Colorado is an 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. A business community as large as downtown Denver has grown in this area, with headquarters for many internationally recognized corporations. These companies indicate Englewood as their location (the default postal place name), although they are actually located in other cities. 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 they live or work in Englewood and identify closely with it may rarely enter the actual city. In Indiana, the ZIP code for a town usually indicates the ZIP code for its corresponding township, as nearly all of Indiana's small town post offices have rural routes.^{[citation needed]}

Acceptable place names also come into play in areas where citizens identify more strongly with a particular urban center than their own municipality. For example, Allegheny County, Pennsylvania, has 130 distinct municipalities, yet many of the county's residents, and even some residents of adjacent counties, commonly use Pittsburgh, PA as their postal address. On the opposite end of the spectrum, in some urban areas, neighborhood names may be acceptable even though they have no legal standing, such as La Jolla, CA, which is the default place name for ZIP code 92037, despite the fact that La Jolla is a district of San Diego, CA and not a separate legal entity (this ZIP code is also in the 919/920 sequence used by San Diego County's suburban and rural areas, not in the 921 sequence used in the remainder of the City of San Diego, even though La Jolla has always been part of San Diego, CA.)

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, New Jersey, but actually serves the entirety of Voorhees Township. This is also the case in LaGrange, New York, a portion of which is served by the 12603 ZIP code based in the neighboring town of Poughkeepsie. The rest of LaGrange is served by the LaGrangeville Post Office. LaGrangeville is itself not a town at all, but a section of LaGrange. Willow Grove, Pennsylvania, served by the 19090 ZIP code, is a village that straddles the border of Upper Moreland Township and Abington Township, and that post office also serves a small portion of Upper Dublin Township. Furthermore, non-municipal place names may also share ZIP codes with municipal place names. For example, West Windsor Township, New Jersey, is commonly referred to in most mailing databases as Princeton Junction, a census-designated place within West Windsor.

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 issues of confusion and lack of identity, some cities, such as Signal Hill, California, have successfully petitioned the Postal Service to change ZIP-code boundaries or create new ZIP codes so their cities become the default place name for addresses within the ZIP code.

Postal designation confusion may have financial implications for local governments, because mail volume is one factor used by the U.S. Census Bureau to estimate population changes between decennial census enumerations.^{[citation needed]} 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 may receive less funds that are computed based on population. A typical example is Paddock Lake, Wisconsin, whose default place name is Salem. Paddock Lake is incorporated as a village within the town of Salem, even though there are more people in the village of Paddock Lake than there are in the unincorporated parts of the town of Salem. Further confusion is caused because Silver Lake, Wisconsin, which is also a village in the town of Salem and is of similar size and status to Paddock Lake, has its own ZIP code and post office.

In another case, the U.S. Federal Communications Commission (FCC) denied a radio station (now WNNX FM) a move requiring a change in its city of license to Sandy Springs, Georgia, largely because it was not a city (until municipal incorporation in late 2005), despite being the seventh-largest place in the state by population. The FCC cited the use of "Atlanta" on letters of support from local organizations, even though the USPS forced them to use Atlanta for 30328 until well after incorporation took effect. Currently "Sandy Springs" is only "acceptable", despite none of 30328 being in Atlanta, or anywhere else outside the Sandy Springs city limit. This even applies to the ZIP code used only for PO boxes at the Sandy Springs main post office.

Because ZIP codes and their associated place names can ignore county lines, problems may occur where street addresses are based on quadrant location within a county. For example, 30339 spans over Vinings, Georgia and Atlanta, Georgia, in southeastern Cobb County; therefore every street address is labeled SE, and has a house number on that county's grid (according to the distance from the town square in the county seat). However, because the USPS demands the use of Atlanta, Vinings addresses are written such that they appear to be in southeast Atlanta, instead of on the opposite (northwest metro Atlanta) side where they actually are.

Division and reallocation of ZIP codes[link]

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.^[12]

In rapidly growing communities, 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.

On the other hand, depopulation may cause a post office to close and its associated ZIP code to be deallocated. For example, Centralia, Pennsylvania's ZIP code, 17927, was retired in 2002.^[13]

Elkins Park, Pennsylvania, was originally issued the 19117 ZIP code, although it lies in Montgomery County. Because of the 191 prefix, which is found only in Philadelphia with that lone exception, auto insurance companies charged higher city premiums to that suburban location. For that reason, residents petitioned the USPS for a 190-prefix ZIP code, which is common to the inner-ring Pennsylvania suburbs of that city, and, after several attempts that were initially disapproved by the USPS, Elkins Park was finally reassigned to the 19027 ZIP code.

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-mayor of Washington, D.C., 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.

Other uses[link]

Delivery services[link]

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.

Statistics[link]

There are over 42,000 ZIP Codes in the United States.^[14] ZIP codes are used not only for tracking of mail but also in gathering geographical statistics in the United States. The U.S. Census Bureau calculates approximate boundaries of ZIP Codes areas, which it calls ZIP Code Tabulation Areas (ZCTAs). Statistical census data is then provided for these approximate areas. The geographic data provided for these areas includes the latitude and longitude of the center-point of the ZCTAs. The Census Bureau provides many statistical data sets for ZIP Codes, but does not keep up-to-date datasets of all ZCTAs. Complete datasets providing a similar approximate geographic extent are commercially available.

ZIP Codes are inherently discrete data, or point based data, as they are assigned only at the point of delivery, not the spaces in between the delivery points. The U.S. Census Bureau then interpolates this discrete data set to create polygons, or areal features representing the approximate extent of the ZIP Code to use for mapping and data presentation. ZCTAs are not to be confused with ZIP codes, and they are not updated as frequently as ZIP Codes. However for many research and planning purposes they are very useful, and can be used with ZIP Code data.

Marketing[link]

The data is often used in direct mail marketing campaigns in a process called ZIP-code marketing. Point-of-sale cashiers sometimes ask consumers their home ZIP code. Besides providing purchasing-pattern data useful in determining the location of new business establishments, retailers can use directories to correlate this ZIP code with the name on a credit card to obtain a consumer's full address and telephone number. 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 (see Elkins Park, Pennsylvania, above.)

According to an online CNN news story by Greg Botelho posted on Thursday, February 10, 2011, "California's high court ruled Thursday that retailers do not have the right to ask customers for their ZIP code while completing credit card transactions, saying that doing so violates a cardholders' right to protect his or her personal information."^[15]

Legislative districts[link]

ZIP codes can be used to identify legislative districts. For example, the website of the U.S. House of Representatives has a "Find Your Representative" feature in the upper left corner based on ZIP code.

Internet[link]

ZIP code data is an integral part of dealer/store locator software on many web sites, especially brick-and-click websites. This software processes a user-input ZIP code and returns a list of store or business locations, usually in order of increasing distance from the center of the input ZIP code. As the ZIP system is confined to the U.S. Postal network, websites that require ZIP codes cannot register customers outside the U.S.^[16]

Credit card security[link]

ZIP codes are used in credit card authorization, specifically Address Verification System (AVS). When a merchant collects the entire address, the ZIP code is an important part of AVS. In some cases, the ZIP code is the only thing used for AVS, specifically where collecting a signature, or other information is infeasible, such as pay at the pump, Redbox terminals, or vending machines.

See also[link]

• J. W. Westcott II
• List of ZIP code prefixes
• Postal Code
• ZIP Code Tabulation Area

References[link]

External links[link]

• ZIP Code Lookup webpage from the United States Postal Service
• ZIP code United States of America webpage with information about ZIP codes of the U.S.

Look up zip, zips, ZIP, or ZIPS in Wiktionary, the free dictionary.

Zip or ZIP most commonly refers to:

• Zipper or zip, a device for temporarily joining two edges of fabric together
• ZIP code, the USPS Zone Improvement Plan used in postal addresses in the USA
• Zip drive, a removable data storage format
• ZIP (file format), a popular data compression and archival format
• zip, a GNU/Linux terminal program from Info-ZIP
• Zip Industries makers of instant hot water dispensers, made in Australia, used worldwide.
• Zip (net-staff) to outsource it; turn it over to be completed; be relieved of it as your virtual assistant will handle it.

Zip, Zips or ZIP may also refer to:

In places:

• Žíp, a village in Slovakia
• Zip City, Alabama, USA

In arts and media:

• Zip, the Dodger, a 1914 film
• Zip (TUGS), a character in the children's television series TUGS
• Mr. ZIP, a promotional character for ZIP codes
• Zip the Pinhead or William Johnson (1842? - 1926), circus freak show performer
• Zip or Tom Prichard (born 1959), professional wrestler
• Zip, the vertical line in works by artist Barnett Newman

In other uses:

• Zips, slang term for Sicilian or Italian immigrants to USA
• Akron Zips, the University of Akron athletic team
• Zip (airline), a defunct Canadian discount airline
• Zip (game), children's or party game
• Zip.ca, a Canadian online DVD rental service
• Zig-zag in-line package, a type of electronic packaging
• Zone Information Protocol, implementation of an OSI model layer
• zip, another name for the convolution operation in formal language theory
• "Zip" is a slang name for zero or no

People with the name[link]

• Bohdan Zip (born 1929), Canadian Politician
• Zip Hanna (1916–2001), American footballer
• Zip Zabel (1891–1970), American baseball player

See also[link]

• ZIP Beep, online humour magazine published 1984-89
• ZIP Skład, Polish hip-hop collective
• Zip and bong, children's or party game
• Zip bomb, a malicious computer file
• Zip card, an electronic ticket for use on London Transport
• Zip-cord, cable for optical communications
• Zip cube, firelighters
• Zip Feed Tower, structure in South Dakota, USA, demolished 2005
• Zip fuel, also called High Energy Fuel
• Zip gun, a makeshift firearm
• Zip-line, a pulley and cable method of descending an incline
• Zip tie, a cable fastener
• Zip tone, in telephony
• Zip's Drive In, American fast-food chain founded 1953 by Robert "Zip" Zuber
• Zipp (disambiguation)
• Zipper (disambiguation)
• Zippy (disambiguation)
• ZIPI

This disambiguation page lists articles associated with the same title. If an internal link led you here, you may wish to change the link to point directly to the intended article.

This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (March 2010)

Morse code, a famous type of code

A code is a rule for converting a piece of information (for example, a letter, word, phrase, or gesture) into another form or representation (one sign into another sign), not necessarily of the same type.

In communications and information processing, encoding is the process by which information from a source is converted into symbols to be communicated. Decoding is the reverse process, converting these code symbols back into information understandable by a receiver.

One reason for coding is to enable communication in places where ordinary spoken or written language is difficult or impossible. For example, semaphore, where the configuration of flags held by a signaller or the arms of a semaphore tower encodes parts of the message, typically individual letters and numbers. Another person standing a great distance away can interpret the flags and reproduce the words sent.

Theory[link]

It has been suggested that this article or section be merged into Coding theory . (Discuss) Proposed since July 2010.

In information theory and computer science, a code is usually considered as an algorithm which uniquely represents symbols from some source alphabet, by encoded strings, which may be in some other target alphabet. An extension of the code for representing sequences of symbols over the source alphabet is obtained by concatenating the encoded strings.

Before giving a mathematically precise definition, we give a brief example. The mapping

Failed to parse (Missing texvc executable; please see math/README to configure.): C = \{\, a\mapsto 0, b\mapsto 01, c\mapsto 011\,\}

is a code, whose source alphabet is the set Failed to parse (Missing texvc executable; please see math/README to configure.): \{a,b,c\}

and whose target alphabet is the set Failed to parse (Missing texvc executable; please see math/README to configure.): \{0,1\}

. Using the extension of the code, the encoded string 0011001011 can be grouped into codewords as 0 – 011 – 0 – 01 – 011, and these in turn can be decoded to the sequence of source symbols acabc.

Using terms from formal language theory, the precise mathematical definition of this concept is as follows: Let S and T be two finite sets, called the source and target alphabets, respectively. A code Failed to parse (Missing texvc executable; please see math/README to configure.): C:\, S \to T^*

is a total function mapping each symbol from S to a sequence of symbols over T, and the extension of M to a homomorphism of Failed to parse (Missing texvc executable; please see math/README to configure.): S^*
into Failed to parse (Missing texvc executable; please see math/README to configure.): T^*

, which naturally maps each sequence of source symbols to a sequence of target symbols, is referred to as its extension.

Variable-length codes[link]

In this section we consider codes, which encode each source (clear text) character by a code word from some dictionary, and concatenation of such code words give us an encoded string. Variable-length codes are especially useful when clear text characters have different probabilities; see also entropy encoding.

A prefix code is a code with the "prefix property": there is no valid code word in the system that is a prefix (start) of any other valid code word in the set. Huffman coding is the most known algorithm for deriving prefix codes, so prefix codes are also widely referred to as "Huffman codes", even when the code was not produced by a Huffman algorithm. Other examples of prefix codes are country calling codes, the country and publisher parts of ISBNs, and the Secondary Synchronization Codes used in the UMTS W-CDMA 3G Wireless Standard.

Kraft's inequality characterizes the sets of code word lengths that are possible in a prefix code. Virtually, any uniquely decodable one-to-many code, not necessary a prefix one, must satisfy Kraft's inequality.

Block codes[link]

This section requires expansion.

Error correcting codes[link]

Codes may also be used to represent data in a way more resistant to errors in transmission or storage. Such a "code" is called an error-correcting code, and works by including carefully crafted redundancy with the stored (or transmitted) data. Examples include Hamming codes, Reed–Solomon, Reed–Muller, Walsh–Hadamard, Bose–Chaudhuri–Hochquenghem, Turbo, Golay, Goppa, low-density parity-check codes, and space–time codes. Error detecting codes can be optimised to detect burst errors, or random errors.

Examples[link]

Codes in communication used for brevity[link]

A cable code replaces words (e.g., ship or invoice) with shorter words, allowing the same information to be sent with fewer characters, more quickly, and most important, less expensively.

Codes can be used for brevity. When telegraph messages were the state of the art in rapid long distance communication, elaborate systems of commercial codes that encoded complete phrases into single words (commonly five-letter groups) were developed, so that telegraphers became conversant with such "words" as BYOXO ("Are you trying to weasel out of our deal?"), LIOUY ("Why do you not answer my question?"), BMULD ("You're a skunk!"), or AYYLU ("Not clearly coded, repeat more clearly."). Code words were chosen for various reasons: length, pronounceability, etc. Meanings were chosen to fit perceived needs: commercial negotiations, military terms for military codes, diplomatic terms for diplomatic codes, any and all of the preceding for espionage codes. Codebooks and codebook publishers proliferated, including one run as a front for the American Black Chamber run by Herbert Yardley between the First and Second World Wars. The purpose of most of these codes was to save on cable costs. The use of data coding for data compression predates the computer era; an early example is the telegraph Morse code where more-frequently used characters have shorter representations. Techniques such as Huffman coding are now used by computer-based algorithms to compress large data files into a more compact form for storage or transmission.

Character encodings[link]

Probably the most widely known data communications code so far (aka character representation) in use today is ASCII. In one or another (somewhat compatible) version, it is used by nearly all personal computers, terminals, printers, and other communication equipment. It represents 128 characters with seven-bit binary numbers—that is, as a string of seven 1s and 0s. In ASCII a lowercase "a" is always 1100001, an uppercase "A" always 1000001, and so on. There are many other encodings, which represent each character by a byte (usually referred as code pages), integer code point (Unicode) or a byte sequence (UTF-8).

Genetic code[link]

Biological organisms contain genetic material that is used to control their function and development. This is DNA which contains units named genes that can produce proteins through a code (genetic code) in which a series of triplets {codons} of four possible nucleotides are translated into one of twenty possible amino acids. A sequence of codons results in a corresponding sequence of amino acids that form a protein.

Gödel code[link]

In mathematics, a Gödel code was the basis for the proof of Gödel's incompleteness theorem. Here, the idea was to map mathematical notation to a natural number (using a Gödel numbering).

Other[link]

There are codes using colors, like traffic lights, the color code employed to mark the nominal value of the electrical resistors or that of the trashcans devoted to specific types of garbage (paper, glass, biological, etc.)

In marketing, coupon codes can be used for a financial discount or rebate when purchasing a product from an internet retailer.

In military environments, specific sounds with the cornet are used for different uses: to mark some moments of the day, to command the infantry in the battlefield, etc.

Communication systems for sensory impairments, as the sign language for deaf people and braille for blind people, are based in movement or tactile codes.

Musical scores are the most common way to encode music.

Specific games, as chess, have their own code systems to record the matches (chess notation).

Cryptography[link]

In the history of cryptography, codes were once common for ensuring the confidentiality of communications, although ciphers are now used instead. See code (cryptography).

Secret codes intended to obscure the real messages, ranging from serious (mainly espionage in military, diplomatic, business, etc.) to trivial (romance, games) can be any kind of imaginative encoding: flowers, game cards, clothes, fans, hats, melodies, birds, etc., in which the sole requisite is the previous agreement of the meaning by both the sender and the receiver.

Other examples[link]

Other examples of encoding include:

• Encoding (in cognition) is a basic perceptual process of interpreting incoming stimuli; technically speaking, it is a complex, multi-stage process of converting relatively objective sensory input (e.g., light, sound) into subjectively meaningful experience.
• A content format is a specific encoding format for converting a specific type of data to information.
• Text encoding uses a markup language to tag the structure and other features of a text to facilitate processing by computers. (See also Text Encoding Initiative.)
• Semantics encoding of formal language A in formal language B is a method of representing all terms (e.g. programs or descriptions) of language A using language B.
• Electronic encoding transforms a signal into a code optimized for transmission or storage, generally done with a codec.
• Neural encoding is the way in which information is represented in neurons.
• Memory encoding is the process of converting sensations into memories.
• Television encoding: NTSC, PAL and SECAM

Other examples of decoding include:

• Digital-to-analog converter, the use of analog circuit for decoding operations
• Decoding (Computer Science)
• Decoding methods, methods in communication theory for decoding codewords sent over a noisy channel
• Digital signal processing, the study of signals in a digital representation and the processing methods of these signals
• Word decoding, the use of phonics to decipher print patterns and translate them into the sounds of language

Codes and acronyms[link]

Acronyms and abbreviations can be considered codes, and in a sense all languages and writing systems are codes for human thought.

International Air Transport Association airport codes are three-letter codes used to designate airports and used for bag tags. Station codes are similary used on railways, but are usually national, so the same code can be used for different stations if they are in different countries.

Occasionally a code word achieves an independent existence (and meaning) while the original equivalent phrase is forgotten or at least no longer has the precise meaning attributed to the code word. For example, '30' was widely used in journalism to mean "end of story", and it is sometimes used in other contexts to signify "the end".

References[link]

See also[link]

Wikimedia Commons has media related to: Codes

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• Equipment codes
• Semiotics
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