why do we need to use city and state in our addresses if ZIP codes are already linked to the city and state?
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It’s a backup. In case the zip code is unreadable for some reason (smudge, bad handwriting, ripped slightly).
It’s possible for a letter to get to its destination with just the zip+4 (which is only 10 or so houses) and a last name. Though it’s a little extra work for the mail carrier.
Some ZIP Codes have more than one city/town. States should automatically pop up, but if the programmer is lazy or company is too cheap to buy this from the USPS, then you have to enter that too.
ZIP codes are a bit more like distribution-center numbers than direct town mappings.
Some smaller towns share ZIP codes, because there are a lot more than 99999 places.
Large cities have multiple ZIP codes, because they have multiple distribution centers.
Some very large volume companies have their own ZIP code, and then the street address doesn’t even matter, since the ZIP code maps to the mailroom already at that address (they do their own sorting/distribution, so they get a distribution center code).
So having the city, state on there helps re-confirm your wishes. Although with automated sorting, the computers just read the ZIP and go with that, you can put random other place names and states with a ZIP code that doesn’t match and it still goes to the ZIP code as long as there is a valid street address “over there” too.
City/State is easiest for humans to process. ZIP is easiest for machines to process.
You may not know the difference between 98101 and 98225 without having to consult an index, but you likely understand the difference between Seattle WA and Bellingham WA with one glance.
Some cities have multiple zip codes and on the flip side, some zip codes cover multiple cities. So, a zip code doesn’t always narrow it down.
Some zip codes cross state lines. If you need to know what state somebody is in you can’t get it 100% of the time from state. In practice that only affects a few.
Towns will be a much bigger problem. Lots of zip codes cover more than one small town. If you have a common street name like Main St, Church St, Park St or Railroad St you’d better include the town name. If zip code 12345 covered 3 small towns they might all have a Main St.
This was in Canada, but about 10 years ago, I wanted to test this.
I wrote an apartment number and a postal code (that I was pretty sure was unique to that building) on an envelope and mailed it.
The US equivalent would be closer to an apartment number and zip+4.
The envelope was received. No name, street, or city.
I now live in the US, and my zip+4 maps to a portion of the floor in my building. I suspect my last name and zip+4 would be sufficient, but I’d be more comfortable with my apartment number and zip+4. But either way, it uniquely identifies my mailbox.
Where I live, a single zip code covers an area of 15km radius, which had almost 4 different villages and 40,000 voters. However, every website I’ve used recently automatically picks the city as soon as I enter the zip code. Just need to enter the exact village I want the package delivered.
this is speculation by me, but I’d assume one of the less prominent reasons is simply that it’s immediately recognizable for a human to read
I know there’s a much more complex (and correct) answer than this, but ZIP codes aren’t 1:1 with cities. My city is a suburban one but has something like a dozen ZIP codes. They provide additional granularity and help route mail more efficiently, though leaving a ZIP code off of your mail will still result in your mail being delivered.
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