In US, can one zip code area be partially in one county and partially in another county? What if the boundary of county changes or counties merge?
Yes, they can even be shared across state lines.
The OP piqued my interest so I looked up how often a county changes boundaries. With the exception of Alaska, and very occasional changes of towns into cities and vice verse, it seems very rare.
I grew up in such a town - the post office town was about three miles away - and in a different county - than our town. It could make for a lot of confusion because our postal address was eastward, but our school district was the town five miles over to the West. Oh, and we lived on a dead end street that started in one town, but we lived on the end - that was a different town - which had a different zip code unless you were one of the ten houses at the end of our dead end street. You don’t even want to know how hard it was to get the road fixed down there or the snowplow to come through. It really did seem like the middle of nowhere.
My zip code covers two counties.
Zip codes were set up for the convenience of the USPS. If it’s efficient to cover more than one county from one location, they’re put in the same zip code.
It’s probably a worldwide problem that zip/postal codes get used for purposes for which they were not designed. In the UK, they affect all kinds of things; especially insurance - neighbours can pay different rates just because they have a different postcode.
Here’s my favorite site for looking up zip code maps and boundaries: http://www.zipmap.net/
The City of st. Louis is independent of St. Louis County in Missouri. In many spots the post office decided to draw zip code boundaries along streets, rather than strictly along the city/county boundary.
This regularly causes confusion because the courts pull their jury lists from zip codes, so people who live along the boundary often get called by the wrong court system.
There are also several zip codes in far west St. Louis County that overlap into franklin and Jefferson counties.
Zip codes often cross county boundary lines. The main basis for the zip code is what area is served by a particular post office. I have lived in three different zip codes that crossed county boundaries that I’m aware of; a fourth one probably did, but I never bothered to look it up to find out.
We have such a street here in Indian Land, SC. To get to the dead end houses, you have to go through another state (North Carolina). Mapping software routinely shows the state boundary deviating around the street, presumably because they can’t believe that what really happens could happen. It’s a painfully long bus ride for those kids. 
Do you have an example? I was of the understanding that that was the one thing they didn’t do.
Check out zip code 38079, as it encompasses the geographic anomaly known as Kentucky Bend, KY along with a lot of Tennessee.
And there are kids who go to high school in Warroad Minnesota who get to go through the Canada on their way to and from school.
Bangladesh and India used to have the worlds worst border for that, but apparently they negotiated it out and now there aren’t parts of Bangladesh in India in Bangladesh in India…
Must be the Angle Inlet kids you’re talking about. At least they can do K-6th grade in their one room schoolhouse.
The small (~5000 people) city I live in is mostly in one county, but a few (~25) houses are in the county to the north(though more are being built)
There is only one zip code
Brian
BTW, it’s ZIP code.
ZIP stands for Zone Improvement Plan
Sure.
I lived in Baltimore City, but had a county ZIP code. It was great for me, because I paid county car insurance rates, which were much lower than city rates.
Yep. Why the U.S. and Canada doesn’t just give the Angle Inlet to Canada and dual citizenship all living residents (and kids who are born to U.S. Citizen parents who want to file the paperwork), beats the heck out of me. It can’t be easy to run that little piece of nowhere.
Was it worse than the Belgian-Netherlands border in Baarle Nassau and Baarle Hertog?
In my town, the ZIP code goes across the boundary between two counties. Irrelevant factoid: The boundary is US Highway 81.
Yes, it was. Far more elaborate/intricate/numerous fragments, including, IIRC, the world’s only 3rd order enclave.
