The ZIP code is the system of postal codes used by the United States Postal Service (USPS). The letters ZIP, an acronym for Zone Improvement Plan, are properly written in capital letters and were chosen to suggest that the mail travels more efficiently, and therefore more quickly, when senders use the code. The basic format consists of five numerical digits. An extended ZIP + 4 code 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.
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 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 sectional center facility, or SCF, also called "sec center." An SCF is a central mail processing facility with those 3 digits. The SCF sorts mail to all post offices with those first 3 digits in their ZIP codes, mail is sorted according to the final 2 digits of the ZIP code and the mail is sent to these 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 during the overnight. 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, thus:
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 enough, the only time the Postal Service issued a stamp promoting the ZIP code, in 1974, Mr. ZIP was not depicted.
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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. Initial attempts to promote universal use of the new format met with public resistance, and today 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 almost 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 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 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 prepended to the front of the box number to produce a four-digit number (e.g., PO Box 52, Garrett Park MD 20896-0052). 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. For a unique ZIP code (explained below), the add-on code is typically 0001.
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 and pre-2007 versions of 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 be placed.)
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 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. 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%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 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.
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. 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 (q.v.).
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. 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.
Geographically, many of the lowest ZIP codes are in the New England region, since these begin with '0'. 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. 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.
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, Iowa, 60601 in Chicago, 77063 in Houston, 80202 in Denver, 94111 in San Francisco, 98101 in Seattle, and 99950 in Ketchikan, Alaska (the highest ZIP code).
The first digit of the ZIP code is allocated as follows:
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.
Despite the geographic derivation of most ZIP codes, the codes themselves do not represent geographic regions; they generally correspond to address groups or delivery routes. Consequently, 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 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.
Similarly, 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", is in Arlington, Virginia.
Rarely, 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. Generally, the locality is so isolated that it is most conveniently served from a sectional center in another state. Examples:
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 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 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 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 Scottsdale city limits.
The result of this is that sometimes people must use the name of a different city than their own. An example is Missouri City, Texas, which is in both Harris and Fort Bend counties and is a suburb of Houston. The portion which is within Harris county is within the zip code 77071, which must use the city "Houston" instead of Missouri City. A tiny portion of the city of Houston is in Fort Bend county in the zip code 77489, and residents there must use the city "Missouri City" for their address even though they are within the city limits of Houston. A former mayor and city council member in Houston lived in Fort Bend counties and were accused of not living in Houston since they had an address of Missouri City.
This phenomenon is repeated across the country. The previously mentioned Englewood 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. 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. 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.
"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. The same thing applies in many cities that have more than one zip code, like Evansville, Indiana or Jacksonville, Florida.
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. 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 Twp. and Abington Twp., and that post office also serves a small portion of Horsham Twp.. Another example is Armstrong Township of Vanderburgh County, Indiana. While the rest of the county uses the 477 prefix, Armstrong Township, despite having no incorporated town, uses the zip code 47617 and addresses itself "Armstrong, Indiana". 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 as Princeton Junction in most mailing databases, despite the fact that Princeton Junction is not a city at all. Instead, it refers to a census-designated area within West Windsor.
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 U.S. Census Bureau 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 fewer State and Federal funds that are computed based on population.
Most significantly, 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.
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.
The above will be made clearer by examining the allocation of ZIP codes in Princeton, New Jersey: