What are postal codes?

Postal codes are strings of numbers (and sometimes letters) that help postal services determine where a piece of mail is being sent to. They help simplify the task of bringing post to its destination. Nations throughout the world use postal codes, and though the systems and formats vary, the common trait is that they all make the postman's job a little easier.

These codes indicate the extent of delivery jurisdictions. Every delivery point that falls under the same jurisdiction is one whole postal code, and anything beyond those dividing lines are counted as a different code. Sometimes postal codes are tied to geographical areas or administrative boundaries. Sometimes they're more free-floating. In some cases, they are attached to a specific organization, or even to a single delivery point. In all cases, they are closely tied to and heavily defined by where the postal employee goes when delivering the mail.

How postal codes are developed and used

Fair warning: we find this subject unreasonably exciting. Accordingly, we're happy to help you find your own postal code. But more than that, these codes had an immense impact on postal operations worldwide. Without them, mail would take longer to reach you. But we're getting ahead of ourselves. First, let's talk about where they came from, and then we can talk about why we have them and how they work.

The source of the code

In the (postal) zone

Back when cities started to get bigger, and reliable mail services became the standard, a problem was identified that made deliveries difficult: In a place like, say, London, there were just too many people for one postman—or even one post office—to deliver to. Like any industry, meeting demand is at least partially a problem of logistics: having the means and the manpower to fill the need. So the first and most obvious answer was to build more post offices and hire more postmen.

Once that happens, though, you start running into a problem. How do you know which houses are the ones you're supposed to take mail to? If Dr. John Watson lives halfway between post office A and post office B, which office should handle his mail? Both could do it, but that would be less efficient; it's smarter to have one postman deliver mail to the Doctor's house and everything to the right of his house, or to have the other postman handle the Doctor's house and everything to the left.

Either way, the work needs to be divided to make sure any given post office isn't doing more than their fair share of labor.

So post offices started drawing lines in the sand. The divisional boundaries they put in place were called postal districts, or sometimes postal zones. To keep track of them all, they were numbered, creating "postal district numbers" or "postal zone numbers."

These postal district numbers started in large cities, and they started a while ago (London, for instance, was divided into 10 districts as early as 1857). WWI Europe had already seen the implementation of similar systems in cities throughout the continent. The US started using them at least as early as 1920. These district numbers were the forerunners of the modern day postal codes.

Post codes from coast to coast—Taking it across the country

The primary difference between these primitive codes and their more evolved cousins is that of coverage: postal districts only happened in the cities, and then only in some cities. They were neither pervasive nor systemic, though the idea of expanded and universal delivery codes started floating around as early as the 1930s. The first country to try and implement a system was the Ukrainian Soviet Socialist Republic (a USSR that eventually became part of the USSR) back in December 1932.

However like everything else that invades Russia during the winter, it didn't find a warm reception. The failed attempt was discontinued in 1939.

Germany had functioning system in place in 1941 despite the war, while the US didn't put ZIP Codes into effect until 1963. Later still, the UK hadn't fully deployed a nationwide implementation until 1974. Since then, though, postal codes have been the norm, with countries expanding their formats to accommodate for additional areas. This is most frequently done by adding digits to the code, a la changing from a 3-digit code to a 5-digit one, and so on.

Today, these codes are used for a great deal. They help with geocoding, address validation, and efficient shipping, to name just a few things. They are even used as part of verifying identity and ensuring that things like credit cards aren't being stolen. So unlike what you learned in high school algebra, these are numbers worth knowing how to use.

International post code names

William Shakespeare once wrote, "A postal code by any other name would smell as sweet." We may have paraphrased that just a little, but the point is, different nations have different names for the same thing. Just like the US calls it a "flashlight" and the UK calls it a "torch," so too are the names of the postal code different depending on where you are. Here are some of the most popular ones:

• ZIP Code: the "Zone Improvement Plan" Codes used in the US
• Postal code: a generic term that can be applied universally; also the name used specifically for Canadian postal codes
• Postcode: a variant of the above; used in multiple English-speaking nations, as well as The Netherlands
• CAP: the acronym for Italy's "Codice di Avviamento Postale", or Postal Expedition Code
• CEP: Brazil's "Código de Endereçamento" Postal, or Postal Addressing Code
• Eircode: Ireland's most recent delivery code system
• PIN code/Pincode: what they call it in India; stands for "Postal Index Number code"
• PLZ: stands for please "Postleitzahl" (which translates to, conveniently enough, "postal code")

Different formats of codes

Formats for these codes vary. There are some cornerstones that can be laid, and some generalities given on the topic, however. Like how they're used heavily by the Universal Postal Union, an agency of the UN.

At first glance

First thing's first: the characters used in the code. Keep in mind that the actual composition of the codes used varies from nation to nation, so not every place is going to use every character. That said, you can expect these three kinds of symbols:

• The ten base characters of the Arabic decimal system (i.e., the numbers 0-9)
• The 26 letters of the basic Latin alphabet (A-Z)
• Hyphens and spaces

Most nations use just the numbers (and sometimes spaces or hyphens, with varying levels of saturation). There are some countries though that like to add a little bit of complexity by throwing in the letters, making the code alphanumeric. Here's the list of who uses these swanky alphanumeric codes, in reverse alphabetical order, because we're cool like that:

• Venezuela
• United Kingdom
• Swaziland
• Somalia
• Peru
• Netherlands
• Malta
• Jamaica
• Ireland
• Canada
• Brunei
• Argentia

Location, location, location

The way postal codes are written is frequently tied to the locations that they are attributed to. It's better this way; rather than just randomly assigning numbers and letters and expecting everyone to remember what they mean, well-intended government organizations group things together for easier labeling.

For instance, in the US the first digit in the code (which can be anything from 0 to 9) indicates what region of the country the destination is in. ZIP Codes that start with "0" are located in northern New England, while ones starting with "9" are tied to the west coast, Alaska, and Hawaii.

Meanwhile, the first 1-2 digits of a UK codes are letters, indicating the city or area of the destination: "L" for Liverpool, "RH" for Redhill, and so forth.

The thing is, the system varies nation to nation (this is starting to seem like a trend). They're not always tied to administrative or geographical borders. Sometimes they're tied to the old postal zones used in big cities. Sometimes they actually are tied strictly to geographical/administrative areas, like provinces and states. Ecuador has a couple of those.

Then there are ones that are close but not quite; sometimes this means codes that never extend beyond a province's borders, but there are more than one code within it.

Many codes are only loosely based on administrative areas, like the US, where the first number gives a reasonable indicator of the destination county. However many ZIP Codes cross county, city, and even state lines.

When it boils down to it, postal codes in this category don't really bother confining themselves to the same boundaries used by the rest of the nation's administrative infrastructure. They put themselves wherever is most convenient for the purposes of running the mail.

Now sometimes, codes won't be tied to boundaries at all; they'll be tied to recipients. Here's a few examples

• Post office boxes often have their own postal code, or postal code suffix (they get their own personal "+4" in the US)
• Some countries use postal codes where each code is unique to the destination address (a little more on this below)
• Large buildings, businesses, and organizations like universities and hospitals often have their own postal code, due to the volume of mail they receive
• Some individuals are such popular recipients that they merit a postal code all on their own: there's a unique post code in Finland (99999), Greenland (2412), and Canada (the H0H 0H0 mentioned above) reserved specifically for Santa Claus (who is known as Joulupukki to the Finnish and Julemanden to Greenland)

The Santa Claus example is the true exception—the others will still be tied to the general geographical location where the destination is.

Postal code accuracy

Divisions of postal jurisdiction notwithstanding, some codes are more accurate than others. By that we mean, some codes point to a post office or a delivery route, while some get specific enough to point to the actual house. In Ireland, the Eircode system launched in 2015 is accurate enough to effectively replace addresses. This wasn't the intention, but the seven-digit codes are each attributed to individual locations.

Theoretically, you could know the Eircode and nothing else, and still make it to the destination.

Most systems don't produce codes that are as accurate as that. US ZIP Codes get to what's called "block level" accuracy; the ZIP+4 system mentioned above denotes the delivering post office with the first five digits, with the additional four indicating the delivery route. That means that everything that shares the same +4 will be hit by the same deliveryman on the same truck during the same trip. Essentially everything he hits on that one delivery run is it's own nine-digit ZIP Code.

Accuracy at that level could mean a single building or organization as mentioned above, or it could mean a street or a block (hence "block level").

Just like everything else, accuracy depends on (you guessed it) the country that developed the code.

How do I use it?

A quick comment about meta-formatting: different countries put the code in different places in an address. While this has nothing to do with the format of the code itself, how it's put together or what it means, it's perhaps more important. You don't necessarily need to know that an Eircode will get you to the doorstep of a property, or what the boundaries of a ZIP Code are. You should probably, however, know where to place it on the envelope (that varies too, by the way).

Most English speaking nations place the code at the end of the address (usually immediately after the name of the city/town). Most European nations place the code at the front of that line—the one that includes the city name—instead of the end. And for a handful of nations (including Japan and the Ukraine), it is listed near the beginning of the address. Long story short, you may want to lookup and verify the address before you start scribbling on the envelope.

Sealing the envelope

Postal codes are an important part of getting mail to its destination. Most countries use them, though yes, the formats differ. What's important to know and to remember is that your postage has a better chance of getting where it's going if you put down the correct postal code. It makes the postman or postwoman's life a lot easier. It's usually the first thing they look at when sorting the mail, and in some cases it may be the last.

And if you're worried about whether the postal code you're using is correct, whether at home or abroad we can help you with that. We can even tell you if the whole address is correct.

So go ahead and do your postal thing.