Everybody knows that every time you mail a letter you have to add a postal code. But did you wondered what is a postal code or what those numbers and letters mean?

This blog post will explain what are postal codes, how they work and why they are super important to make sure you mail is always delivered on time.

How Postal Codes are Developed and Used

Postal codes, also known as ZIP Codes in the U.S., facilitate the automated sorting of mail.  The wide adoption of business mail was one of the key reasons people use postal codes. However, USPS introduced them in 1943, for a different reason.

During the WW II, many postal workers served the military. As a result, the whole system became understaffed. Thus, USPS had to come up with a method that would make the mail sorting a lot easier for newer and far more less experienced employees.

So, USPS classified the largest US cities with two numbers. They used the first number to identify the city. As you might have imagined, the second number was used to identify the state.

However, the mail volume continued to grow and, in 1963, they added three new numbers.

To sum up, here is what the first five numbers mean:

  • The first number identifies an area. For example, 0 stands for the North East and 9 for the far West.
  • The next two digits uniquely point to the central post office in the said region.
  • The two digits at the end, refer to a postal zone or a small postal office

While useful, these 5 digit codes weren’t enough to keep up with the increased number of mails. So, in 1983, USPS introduced the ZIP+4 codes:

Let’s see how these numbers help:

  • The 6th and the 7th represent something called a “delivery sector”. This might be a large building or even a small geographical area.
  • The last two digits refer to a “delivery segments”. Delivery segments can narrow down the delivery area to a floor of a building.

Different Formats

Post Office

So far, we’ve explained the US Postal Code format. However, each country uses its own format. Also, people don’t use the name “postal code” everywhere. Even if it used to be a generic term, now it is mostly used for Canadian postal codes. Here are the most well-known alternatives:

  • ZIP Code, which stands for the “Zone Improvement plan”, is used in the US
  • CAP, which stands for “Codice di Avviamento Postale”, is used in Italy
  • Postcode, popular in many English-speaking countries
  • Eircode, the equivalent system in Ireland.

As we’ve already mentioned, these codes don’t share the exact format. Even if the Universal Postal Union, which is the specialized agency of the United Nations (UN), laid out some guidelines, each country developed its own system. Let’s take a closer look:

  • Usually, postal codes are comprised of the ten digits of the decimal system, that is 0 to 9.
  • Some countries, like the United Kingdom, Canada, or the Netherlands do use alphanumeric codes. So, these codes are a bit more complex.
  • Even if the postal codes are tied to a specific location, the way this is implemented varies from country to country. The US uses the first digit to identify an area and the next to point to a central post office. At the same time, a UK code starts with two letters that are indicating a city or an area.
  • A few well-known individuals have their own postal codes. Take Finland’s 99999 as an example. Do you know to whom it belongs? Let me answer: it is Santa Claus’ personal postal code. I know, we should have published this blog post a few weeks ago. Anyway, keep this in mind. It might prove useful next year.


As you might have guessed, not all postal codes provide the same level of accuracy. For example, Ireland’s Eircode point to an individual location. Meanwhile, a US ZIP+4 code is able to narrow down the delivery area to a single building or a specific floor of a building.

Also, each country has its own way of putting the code in a specific place in an address. As a result, you have to know where to place the postal code on an envelope:

  • The vast majority of English countries put the postal code after the name of the town. That is at the end of the address
  • Conversely, European countries place the postal code before the name of the town.

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How Postal Codes are Developed and Used

Accurate postal codes are a key factor. They assure that the mailer always deliver your mail to its destination. Their role is to significantly ease the sorting and delivery of email. Also, in a few cases, a mailer can refuse to deliver a mail with an incorrect postal code.

Happily, there is no need to worry. Our WinPure Clean & Match Enterprise with Address Verification is here to help and correct all the postal codes you use.  Whether its in the US, Canada or UK,  the software covers over 240 countries worldwide. Try it today and discover why large companies such as Bank of America, Vodafone, McAfee, Hewlett-Packard, and Emirates are happy to use our software.

By Andrei Popescu | December 27th, 2018 | Posted in Address Verification

About Andrei Popescu

Andrei is a detail-oriented writer and enjoys the process of researching and learning new things all data quality.

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