alt Hacker NewsUSPS doesn't care; each ZIP code has a single preferred city name and a list of acceptable alternate city names to account for cases in which a ZIP code spans multiple cities. However, USPS's address validation will prefer to use the preferred city name for the ZIP regardless of whether the recipient actually lives within the boundaries of that city. That's because USPS has opted to organize addressing entirely around the ZIP codes, and other political boundaries are irrelevant except in cases of problems interpreting the address.
This does mean that you might autofill a city name that is "wrong" in the view of the person completing the address form, but much of the bulk mail they receive probably uses that city name anyway.
Technically speaking ZIP codes are not "supposed" to span states but, in exceptional cases, some do. In this case USPS handles it the same way: the state of the preferred city is the preferred state for the ZIP code.
The preferred city is almost always the location of the post office serving the ZIP, which makes this situation fairly intuitive. You can find some interesting edge cases where a post office in located in a suburb city, resulting in a ZIP that includes part of a major city having the suburb as its preferred city name.
You can look up the city name and alternates for a ZIP here: https://tools.usps.com/zip-code-lookup.htm?citybyzipcode and the Domestic Mailing Manual covers this, although it's scattered across several sections and mostly part of how the City State database (the database used for validating city and state names in addresses) works.
Replies
> Technically speaking ZIP codes are not "supposed" to span states but, in exceptional cases, some do. In this case USPS handles it the same way: the state of the preferred city is the preferred state for the ZIP code.
I've heard this before but it raises a million questions for me and I don't understand how this doesn't cause massive systematic problems and headaches in practice. Are residents even usually well-aware what city they live in, versus what's on their postal address? I sure as heck have always assumed whatever my mailing address says is the city I live in; I can't imagine a ton of people questioning it.
Doesn't this mean a ton of citizens would be registering for the wrong state's elections? Do the election officials always catch these? What about businesses - don't they constantly pay taxes incorrectly if the address is written incorrectly? What about laws (say privacy, wiretapping/call recording, etc.) where people make assumptions based on the city and state - what if they're wrong because the written city isn't the actual city? Who's criminally liable then?? Does every business have to perform a jurisdiction lookup to make sure an address isn't misleading?
I agree using the preferred city name works just fine for USPS, though maybe not for UPS/Fedex.
What I want to know is: Why isn't this preferred city+state mapping dataset for zip codes publicly available from USPS? It would be like 40kb of data for the entire thing. Why is this not public domain from the US Government?
Edit: or is this what I'm looking for (the "Physical City", "Physical State" columns? https://postalpro.usps.com/ZIP_Locale_Detail
It's missing 00501 at least (which zippopotam has), and military zip codes (which zippopotam doesn't have). Military zip codes are included in this file: https://postalpro.usps.com/areadist_ZIP5
Also fun fact 88888 is for "Operation Santa" uspsoperationsanta.com, which zippopotam is missing, but appears in the areadist_ZIP5 file.
But addresses aren’t just for sending mail. Location also determines which municipal and state laws apply, so there are contexts where the distinction matters.