Science Fair Project Encyclopedia
A ZIP Code is the postal code used by the United States Postal Service, which always writes it with capital letters. ZIP is an acronym for the Zoning Improvement Plan, but was also cleverly meant to suggest that mail travels more efficiently (and therefore faster) when senders use it. The basic ZIP Code format consists of five numerical digits. An extended ZIP+4 code includes the five digits of the ZIP Code plus four digits which allow a piece of mail to be delivered to a specific address. ZIP Code was originally registered as a trademark by the U.S. Postal Service, but its registration has since expired.
The postal service implemented postal zones for large cities in 1943. For example:
John Smith 3256 Epiphenomenal Avenue Minneapolis 16, Minnesota
The "16" is the number of the postal zone within the city.
By the early 1960s a more general system was needed, and on July 1, 1963, non-mandatory ZIP Codes were announced for the whole country. Robert Moon, an employee of the post office, is considered the father of the ZIP Code. He first submitted his proposal in 1944 while working as a postal inspector . The post office only gives credit to Moon for the first 3 digits of the ZIP Code, which describe the region of the country.
In most cases, the last two digits of the ZIP Code coincide with the older postal zone number, thus:
John Smith 3256 Epiphenomenal Avenue Minneapolis, Minnesota 55416
In addition, two-letter abbreviations were introduced for states, eliminating the need to write the state's name out in full. For example, California is CA. Abbreviations are also assigned for U.S. territories like Puerto Rico (PR) and American Samoa (AS), as well as for several former U.S. Trust Territories in the Pacific, such as the Federated States of Micronesia (FM), which are now separate countries.
Similarly, US military addresses also have their own abbreviations. Mail to these addresses is sent to the Army (or Airforce) Post Office (APO) or Fleet Post Office (FPO). This may also be used for mail to many US diplomatic missions overseas.
In 1983, the US Postal Service began using an expanded ZIP Code system called "ZIP+4", which are often called "plus-four codes". A ZIP+4 code uses the basic 5-digit ZIP plus an additional 4-digits to identify a geographic segment within the 5-digit delivery area, such as a city block or a group of apartments or an individual high-volume receiver of mail, or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery. Use of the plus-four code is not required, but it helps the Postal Service direct mail more efficiently and accurately because it reduces handling and significantly decreases the potential for human error and possibility of misdelivery, and thus reduces the cost of delivery.
Postal bar code
The ZIP Code is often translated into a barcode called POSTNET, that is printed on the mailpiece as well, to make it easier for automated machines to sort the mail. Unlike most barcode symbologies, POSTNET uses long and short bars, not thin and thick bars. The barcode can be printed by the person who sends the mail, or the post office will put one on when they receive it. If the post office does it, they either have a machine OCR it, or have a human read the address if absolutely necessary. (The automated machinery has the unfortunate tendency to paste the coding over the bottom half-inch of postcards, often obliterating the signature.)
People who send bulk mail can get a discount on postage if they have pre-printed the barcode themselves. This requires only a simple and often free font, and the knowledge of at least the main 5-digit code, if all 9 are not available. An additional two digits are usually used to indicate the exact delivery point, so that every single mailable point in the country has its own 11-digit number. These two digits are usually the last two of the street address or box number, though non-numeric points with names or letters are assigned DP numbers by the local post office. The last digit is always a check digit, which is obtained by adding up the 5-, 9-, or 11-digits, then subtracting the last digit of that result from 10. (Thus, the check digit for 10001-0001 00 would be 7, or 1+1+1=3 and 10−3=7.) The sender needs only to type something like /100010001007/ in the 12-point POSTNET font to create the code for printing.
Structure and allocation
ZIP Codes are numbered with the first digit representing a certain group of U.S. states, the second and third digits together representing a region in that group (or perhaps a large city), and the fourth and fifth digits representing more specific areas, such as small towns or regions of that city. The main town in a region (if applicable) often gets the first ZIP Codes for that region; afterwards, the numerical order often follows the alphabetical order.
Geographically, many of the lowest ZIP Codes are in the New England region, since these begin with '0'. Also in the '0' region are Puerto Rico and the US Virgin Islands. Some low zip codes are: 00501 for Holtsville, New York (a unique ZIP code for the U.S. Internal Revenue Service center there); 00601 for Adjuntas, Puerto Rico; 01001 for Agawam, Massachusetts, and 01002 for Amherst, Massachusetts.
The numbers increase southward along the East Coast, such as 10036 (New York City), 20500 (Washington, DC), 30303 (Atlanta, Georgia), and 33130 (Miami, Florida). From there, the numbers begin increasing heading westward and northward. For example, 40202 is in Louisville, Kentucky, 50309 in Des Moines, Iowa, 60601 in Chicago, Illinois, 75201 in Dallas, Texas, 80202 in Denver, Colorado, 94111 in San Francisco, California, 98101 in Seattle, Washington, and 99950 in Ketchikan, Alaska.
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)
- 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)
- 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)
Other U.S. territories have codes starting with 9. The next two digits represent the sectional center facility (sorting area for a region) (e.g. 432xx = Columbus OH), and the fourth and fifth digits represents the area of the city (if in a metropolitan area), or a village/town (outside metro areas): 43209 (4=Ohio,32=Columbus,09=Bexley). When a sectional center facility's area crosses state lines, that facility is assigned separate three-digit prefixes for the states that it serves; thus, it is possible to identify the state associated with any ZIP code just by looking at the first three digits.
Like area codes, ZIP Codes are sometimes divided and changed, especially when a rural area becomes suburban. Typically, the new ZIP codes become effective once announced, and a grace period (e.g., one year) is provided in which the new and old ZIP codes are used concurrently, so that postal patrons in the affected area can notify correspondents, order new stationery, etc.
Most significantly, in rapidly developing suburbs, it is sometimes necessary to open a new sectional center facility, which must then be allocated its own three-digit ZIP-code prefix or prefixes. Such allocation can be done in various ways. For example, when a new sectional center facility was opened at Dulles Airport in Virginia, the prefix 201 was allocated to that facility; therefore, for all post offices to be served by that sectional center facility, the ZIP code changed from an old code beginning with 220 or 221 to a new code or codes beginning with 201. However, when a new sectional center facility was opened to serve Montgomery County, Maryland, no new prefix was assigned. Instead, ZIP codes in the 207 and 208 ranges, which had previously been assigned alphabetically, were reshuffled so that 207xx ZIP codes in Montgomery County were changed to 208xx codes, while 208xx codes outside that county were changed to 207xx codes. Because Silver Spring (whose postal area includes Wheaton) has its own prefix, 209, there was no need to apply the reshuffling to Silver Spring; instead, all mail going to 209xx ZIP codes was simply rerouted to the new sectional center facility.
By type (i.e., use)
There are three types of ZIP codes: unique (assigned to a single high-volume mailer), PO box only (used only for PO boxes at a given facility, not for any other type of delivery), and standard (all other ZIP codes). As examples of unique ZIP codes, certain governmental agencies, universities, businesses, or buildings that receive extremely high volumes of mail have their own unique ZIP Code, such as 81009 for the Federal Citizen Information Center (FCIC) of the U.S. General Services Administration (GSA)  in Pueblo, Colorado, 21250 for University of Maryland Baltimore County, 30385 for BellSouth in Atlanta, and 10048 for the World Trade Center complex in New York, New York (until its destruction on September 11, 2001). The White House has its own secret ZIP Code, separate from the publicly-known 20500, for the President of the United States and his family to receive private mail. An example of a "PO box only" ZIP code is 22313, which is used for PO boxes at the main post office in Alexandria, Virginia. In the area surrounding that post office, home and business mail delivery addresses use ZIP code 22314, which is thus a standard ZIP code.
The above will be made clearer by examining the allocation of ZIP codes in Princeton, New Jersey:
- 08540, standard (deliveries in most of the Princeton postal area)
- 08541, unique (Educational Testing Service)
- 08542, standard (deliveries in the central area of the borough of Princeton)
- 08543, PO box only (PO boxes at the main post office)
- 08544, unique (Princeton University)
Delivery services other than the USPS, such as Federal Express, United Parcel Service, and DHL require a ZIP code for the optimal internal routing of a package. This spares customers from being required to use some other routing designator, such as the IATA code of the destination airport or railhead.
ZIP Codes are used not only for tracking of mail, but in gathering geographical statistics in the United States. The U.S. Census Bureau keeps track of the latitude and longitude of the center-point of each ZIP Code, a database which numerous other companies sell. The data are often used in direct mail campaigns in a process called ZIP Code marketing , developed by Martin Baier . ZIP-coded data is also used in analyzing geographic factors in risk, an insurance industry and banking practice pejoratively known as redlining.
U.S. Postal Service codes
Postal Codes in other countries
- ZIP Code lookup from the United States Postal Service.
- Java Applet Showing US ZIP Code Distribution
- ZIP Code Download Free ZIP Code Lookup.
- Zip Code - Zip Codes - All States Alphabetic and Numeric Zip Codes
- ZIP Code Subscription Service
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