Just over a half-century ago, in 1963, the United States Postal Service introduced its five-digit ZIP code system to make mail more efficient, but it was an updated idea on something the postal service began during World War II. By 1983, the U.S. Postal Service would try to introduce the four-digit ZIP code add-on, but it wouldn’t go well. These are just some of what makes the U.S. ZIP code system a fascinating rabbit hole to go down.


Nope. You don't need to use the extra four-digit ZIP code. In fact, you don't need to use a ZIP code at all, but they help your mail get there a whole lot faster, so use them if you've got 'em.

Brief History of ZIP Codes

The five-digit ZIP code was brought in back in 1963, yes, but it was initially preceded by the city code. The name ZIP stands for “Zone Improvement Plan,” and it was the birth of an ambitious attempt to make mail delivery more efficient in 1943 when an overwhelming number of mail workers left to join the World War II military.

For 124 of America’s biggest cities, a city code was implemented to help make it easier to hand-sort the mail into the right delivery catchment areas. So, you might have seen an address that said “New York City 4, NY,” which gave the sorters a specific neighborhood in New York City so they could pass it off to those sorters.

Back in 1943, the American population stood at 136 million people, and by the time 1963 rolled around, 189 million people were living in the United States. With that massive increase in population came a lot more mail, so that’s when an idea that postal supervisor Robert Moon submitted in 1944 spurned an innovation that would take the USPS to new realms of efficiency. But Mr. Moon’s idea represented just the first three numbers in the ZIP code – the last two digits were the brainchild of a committee that worked on the idea.

So What’s in a Number?

Obviously, ZIP codes by city are a thing, but you could have your own ZIP code if you’ve got a distinct enough location or address. ZIP codes are all about narrowing down a delivery location, and if a business or person gets enough mail, a dedicated ZIP might just be worth it from the USPS’s perspective. You can apply for a four-digit ZIP code addendum through the USPS website.

But how does the ZIP code itself work? Area ZIPs seem to start similarly, so the numbers must mean something, right? Of course! Say you have one of the United State’s most recognized ZIP codes – the famous 90210 – the numbers provide a lot of information.

  • The first digit, 9: This is the “national area” for the mailing code. The 9 is for anywhere on the West Coast and the Pacific, including Guam and American Samoa. 900 to 961 are all located in California, for example.
  • The next two digits, 02: This is the large city post office. In Los Angeles, “02” covers Compton, Culver City, Venice, Marina Del Rey and a few other areas.
  • The final two digits, 10: This denotes the associate post office or delivery area. Culver City, for example, is served by 90230, 90231, 90232 and 90233.

The Four-Digit ZIP Explained

Officially called the “ZIP+4 code,” these last four digits round out a sorting system that’s the difference between landing your dart on the dartboard and hitting the bull’s-eye.

  • The sixth and seventh digit: These denote a specific street or large building, such as an apartment block with 400 apartments that has a large volume of mail.
  • The eighth and ninth digits: These dictate which side of the street the address is on or even a specific floor of the large building designated by the preceding two numbers.

Nice to Have, Not Needed

The four-digit ZIP was rolled out in 1983 to help the postal service sort mail faster. Americans didn’t really dig the idea of making more work for their address, and most never bothered to use the code. As the old country adage goes, you can lead a horse to water, but you can’t make it drink. So, the USPS stopped trying to convert a reticent public, and today, the four-digit ZIP code is nice to have but isn’t required.

That said, if you have the extra digits, use them! They really will help your mail reach its destination a bit faster. Think of the five-digit ZIP code as knowing you have to go to Fenway Stadium but not knowing which part exactly. The four-digit add-on is similar to getting a ticket to a ball game, which tells you what section and seat you’re in.

But That’s What Addresses Do…

Yes and no. Addresses are great, but they still take just a bit longer than sorting a number does. Plus, there are so many similar names – Maple Court, Maple Lane, Maple Street – and instead of requiring the brainpower to sort those out, that extra four-digit code does it with ease. Think of all those individual letters daily – imagine how much time each of those "split-second" sortings might save over the course of eight hours or a year.

Luckily, computers and sorting technologies evolved at the same time that the four-digit code came out. So, while it would have been nice for the American public to adopt the code, technology picked up the slack and helped achieve the same result.

And Yet, It’s Not Needed?

That’s right. You don’t need to include the four-digit “ZIP+4” code unless you feel like it since technology has you covered these days. But there’s more: You don’t even need to put the ZIP code itself, but you’re slowing down your letter’s progress if you don’t.

How the “National” Code Works

What’s neat about the ZIP code’s first number is that it’s pretty handy for a rough geographical area. The code goes from 0 through 9 for the first digit, with the “0” zone being the northeastern-most part of the U.S. – essentially New England. The “1” is for New York and Pennsylvania. Then, it’s down the coast for codes “2” and “3,” which takes you down to Florida, Georgia, Alabama and surrounding states.

From “4” through “6,” it’s the American Midwest states. Codes that start with “7” include Texas, Oklahoma, Arkansas and Louisiana. And if the “9” codes are the Pacific Coast, that means “8” denotes Nevada, Utah, Colorado, Arizona, Wyoming, New Mexico and Idaho.

Postal codes are used by countries around the world, and one of the first places they occurred was in the United Kingdom where London had divided the city up into 10 postal zones by the 1860s. In fact, the U.S.'s city code was in use in places by the 1920s but wasn't nationalized until World War II.