Understanding US Postal ZIP Code Zones

By Eva Latch
Updated July 29, 2016
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Understanding US Postal ZIP Code ZonesZip codes are used by the U.S. Postal Service (USPS) in order to make sure mail gets where it should. The term "ZIP" stands for 'zone improvement plan' and all zip codes have a basic format, which is five digits. However, in 1983, changes were made, which resulted in the introduction of the ZIP + 4 format. Hence, US postal zip code zones now have five digits and a hyphen followed by a further four digits.

Understanding US Postal Zip Code Zones:

Zip codes were first used in 1943. At that point, the USPS decided that larger cities inside states should be recognized by a code. Robert Moon, a postal worker, came up with this idea and developed a proposal in 1941. In the early 60s, a more general system was adopted, made available for the entire country in 1963.

In the original Moon proposal, each of the sectional center facilities in the country should use three digits to identify them. This is still used today. The five numbers of the zip are firstly the sectional facility number, and then the US postal zip code zones, referring to the geographical area. So, if we take Aaronsburg, PA, as an example, their zip code is 16820, where 168 is the sectional facility, and 20 represents Pennsylvania.

When the zip codes were adopted nationwide in 1963, they were only applied to first class mail. Packages, second, and third class post joined in 1967. The ZIP + 4 system that is used today provides further details of a more specific geographic location. Post office sorting machines transfer the code to a bar code, which ensures that it is sent to the right locality. The +4 component of the code also shows what the post office box number is, if applicable. Post office boxes, however, do not have a uniform code. They require a 12 digit zip code instead.

Out of the five numbers, the first is a geographical area, such as the North West. The second and third refer to the specific facility. Digits four and five focus on a more specific region. In most cases, the region's biggest city receives the first zip code. All surrounding cities and towns get theirs alphabetically.

The very lowest numbers of zip codes are found in New England. They all start with the number 0. The Internal Revenue Service also has the number 0, however, and so with Puerto Rico's Adjuntas, as well as any other cities and towns whose name starts with an A. New York State has been given the number 1 initial digit. Hence, all New York zip codes start with a 1.

Moon's zip code idea was quickly adopted all over the world. However, other countries generally use different types of codes, referring not only to specific postal sorting facilities, but also to actual street names. Different countries have different ways of achieving this, although almost all use some sort of alphanumerical code system that shows wider geographical area, all the way down to a smaller geographical area and even the specific house.

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