Today, we’re going to start with rules but go heavy on history at the end because what I thought would be an easy question to answer took me down an interesting rabbit hole and finally to an interview. It all started with this voicemail message.

“Hi, Grammar Girl. I actually work for the Federal Government, and I have a good enough command of writing and grammar that I am humorously referred to as the Grammar Guru. Recently we were writing a report and something came up. Some of the people on the committee thought we should capitalize ‘ZIP Code’ . . . and my take on that is that would be correct if you are referring to the specific thing — the postal system, that specific system — but if you’re talking about ‘zip code,’ it has become kind of a generic, like kleenex. I don’t listen to your podcast, so if you decide to use this, could you please answer it in your column. Thanks very much, Grammar Girl. I remain a faithful reader. Thanks again. Bye bye.”

Well, first, you should listen to the podcast, but the answer will be on the website as an article too. It’s extremely rare that material isn’t on both. But thank you for submitting the question as a voicemail so I can use in the podcast!

As I said, I thought this would be a quick answer: I’d look it up in a couple of style guides and bing-bam-boom, the end. But it wasn’t so simple, and I started finding interesting tidbits.

What Are ZIP Codes?

For our international readers, ZIP codes are an American thing. They’re the five digit codes at the very end of a mailing address (and if you want to be really detailed, you can add four more numbers to the end, and in my experience, doing that does help the post office deliver your mail a little faster). Other countries sometimes call the address codes “postal codes,” and some countries don’t have them at all. In the United States, we use just numbers, but some other countries also use letters.

Since our caller works for the U.S. government, and ZIP codes are a government thing, I checked the US Printing Office style guide first. It recommends “ZIP Code.”

Associated Press style is similar: “ZIP code”—two words with “ZIP” in all caps—but it uses a lowercase C, and I was surprised it would deviate.

I looked a little deeper, and this made me laugh: Someone asked in the AP Stylebook Q&A section why AP style lowercases the word “code” when the source, the US Postal Service, capitalizes it, and the reply from the AP editors was “The U.S. Postal Service likes capitalization more than we do. We have different styles.” So there you go! They just like it better lowercase. That’s how styles work.

‘ZIP Code’ Was Trademarked

I didn’t know “ZIP” stands for “Zone Improvement Plan,” and the US Postal Service originally trademarked the phrase “ZIP Code.”

When I look at the listing on the US Patent and Trademark Office website, it looks like that trademark expired in 1997. On the other hand, the US Postal Service website still puts a “™” after “ZIP Code.” So maybe I missed something, or maybe whoever writes their website didn’t get the memo.

The AP Stylebook doesn’t put a trademark symbol after “ZIP code,” but it doesn’t recommend doing it for any trademark, so that doesn’t tell us anything about the trademark status.

I also found that the home furnishing company Wayfair holds the trademark for use of the term “zipcode” (all one word) on items like lamps, carpets, bedding, and furniture. How weird is that? So I searched the site to see if they had products with the word “zipcode” on them, but they don’t. Instead they have what looks like a house brand called Zipcode Design. So that makes more sense.

Check 3 Style Guides and Find 3 Different Ways to Write ‘ZIP Code’

The three style guides I checked each have a different way of writing “ZIP code.”

? The US Government Publishing Office uses “ZIP Code.”
? The AP Stylebook uses “ZIP code.”
? The Chicago Manual of Style uses “zip code.”

‘Zip-code’ Is a Verb

You thought we were finished, but “zip-code” can also be a verb! Both, Dictionary.com and Merriam-Webster list “zip-code” as a verb, and they keep it all lowercase and use a hyphen. It means to mark something with a ZIP code, as in “Be sure to zip-code that letter.”

The History of ZIP Codes

ZIP codes are much newer than I thought they would be.

The main 5-digit codes we use today in the United States were only introduced in 1963, and at first they were optional. I’m pretty sure that even today, some people don’t have one, and that’s when I realized that I know someone who knows all kinds of interesting things about the history of addresses. She’s Deirdre Mask, the author of the new book “The Address Book: What Street Addresses Reveal about Identity, Race, Wealth & Power.” Click the player at the top of the page to listen to the interview with Deirdre Mask (it starts around the 7:15 mark) or read a transcript of the interview.

Image courtesy of Shutterstock.