[chromatin]

My ZIP code happens to be shared by two separate cities and there are a few websites (Github, I'm looking at you) that will fail the payment, registration, etc. attempt if you don't enter the municipality that it THINKS is correct.

[ProllyInfamous]

Exact opposite problem at my former rental: Two different properties with different ZIP codes... but they had the same address on the same road, just a mile apart (different jurisdictions).

I lived in a house; the other location was a nail spa. Strangers sometimes visited thinking they were at the right address (they weren't) to get their nails'did (they didn't).

[dhosek]

A lot of this is on the people doing the platting in the first place. I would note, for example, that where I live in Oak Park, when they replatted the suburb after splitting from Austin Township with a different grid than is used in Chicago and the neighboring suburbs that

1. They kept the Chicago grid on the edge streets of the village so that, e.g., 110 North Austin would be across the street from 111 North Austin and

2. If they had kept the usual new 100 at each block system, the north-south streets on the south end would have been 1200–1249 which would have been identical to the numbers of the next block south in Berwyn and Cicero so the last block on the north-south streets is instead 1150–1199.

Contrast the borders of Los Angeles which in some areas are almost fractal in their complexity (there are buildings which straddle the boundary between L.A. and its neighbors and many blocks where adjacent buildings are in different cities). For whatever reason, the powers that be decided that the incompatible address numbering between adjacent cities should be retained so you will have weird discontinuities in building numbering along a block depending on what city the building lies in. I remember my wife having a doctor’s appointment in a building which was one of those which crossed the border so it had two different addresses assigned to it, one for Los Angeles and one for Beverly Hills.

[dylan604]

> to get their nails'did (they didn't)

I'm thinking an opportunity was missed here.

[dfxm12]

Would ZIP+4 help here?

[stackskipton]

Yep, Zip + 4 is pretty pinpoint, generally it's no more than 20 residences and there is plenty of Zip + 4 that is single address.

[dhosek]

A lot of P. O. Boxes have their own zip+4 address. You can spot these because the P.O. Box number and the +4 are usually the same (or occasionally the P.O. Box number is the last two digits of the five-digit zip code combined with the +4 so that a piece of mail with a box number and no zip code can be delivered to the correct post office in a large city).

[stackskipton]

Yea, PO Boxes generally have their own Zip Code so they use +4 for that.

However, if you look at zip+4 for dwellings, it’s still few. My cul de sac with 5 houses has zip + 4 different from house on connecting street.

[dhosek]

It’s based at least in part on delivery routes.

There’s a less-known 11-digit zip code which is unique for every delivery point (so down to the individual residence). I’m not sure if multiple apartments in the same building have distinct 11-digit zip codes, but this does imply that a zip+4 cannot have more than 100 delivery points within its bounds.

[stackskipton]

Apartments will commonly have multiple Zip + 4 for the building so I imagine each apartment gets unique Zip 11.

Open Google Maps, go to Central Park in NYC, search for apartments and randomly pick one. Then go USPS Zip Code lookup (https://tools.usps.com/zip-code-lookup.htm) and punch in the address leaving off any apartment number so it will show all available addresses. I used 225 E 63rd St New York, NY. Appears they have 8 Zip + 4 assigned to the complex.

I tried again with building in Philly and same story. Each floor of 16 apartments got its own Zip + 4.