Why use Zip+4 numbers for P.O. boxes?

Every time I’ve seen it done, it just repeats the last four digits of the P.O. box anyway. What’s the point?

If the box number can’t be read, the Zip + 4 lets you deliver it properly.

In other words, if the address is

Jim Smith
P.O. Box <unreadable>
Podunk, NT, 02345-0128

It gets delivered to box 128.

In some cities, there were duplicate series of PO Boxes at the different ‘satellite’ post offices in town. So your PO Box 14 in, say, Houston might have been at one post office, 77005-0014, while someone else might have PO Box 14, also in Houston, 77026-0014.

I don’t receive mail at home, only at the PO Box. I don’t generally use the last four digits of the zip code (which are identical to the PO box number in my case). But when ordering items to be delivered or requesting refunds, I find that some companies require me to list a street address, in addition to or instead of the PO box number. In those cases, I’m careful to add the last four digits so that the postal service won’t try to deliver it to a nonexistent rural-route box.

When I lived on a very very very rural backroad in the middle of nowhere NY there was a +4 for our mailbox outside. I still remember it, though I don’t think I ever had to use it.

Hell, the mailman lived a mile away and knew us well… If it made it to town with my last name on it and nothing else he’d just bring it to me, no need for +4.

What would be the point of a +4 in a mailing route?

I was under the impression the last four were for the individual house/building.

Substitute mail carriers. Doesn’t your carrier ever have a day off or go on vacation? Trust me, some substitutes do not, in the witty recently coined Pit phrase, know their burro from a burrow.

No, not quite. “A ZIP + 4 code uses the basic five-digit code plus four additional digits to identify a geographic segment within the five-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery.” From the Wiki article on zip codes.

That’s a good argument for my we use the first five for post office boxes, but why the last four which is what the OP is wondering.

The last four digits in the barcode of the zip code allows the Delivery point sequencing letter sorters to place the mail into delivery sequence. When a tray of mail is in numerical order, it is a lot more efficient for the clerks to case it into the boxes then if it was just haphazardly in any random order.

Well all 30 houses on the whole route with names couldn’t confuse the new guy so much that he needed an extra zip code plus the name on every box… Probably one of those universal things where every small town and big city is required to have it…

I’ve seen many cases where the first of the four digits differs.

Are US zip-codes precise enough to be used in sat-navs?

In the UK, because each post-code only covers a very small area, it’s possible to enter the post-code plus house number and it will take you straight to the correct front door.

[del]The Post Office Box I use for my non-profit group has a +4 number that doesn’t match the last four digits of the P.O. Box. This is a P.O. Box at a US Postal office.[/del]

Never mind - I just checked with the zipcode finder at usps.com and I realize that the clerk had told me wrong when we opened it all those years ago. The +4 does match the last four digits of the P.O. Box.

The +4 for my P.O. Box is not the same as the last 4 digits of the box number.

Are there large post offices that have more than 10,000 boxes? Well … I guess I really mean 9999 boxes because I doubt if anyone is box 0. Four digits isn’t unique then so would they have another different zip code?

I don’t know if there are more than 9999 P.O. boxes at the post office where my non-profit group has theirs, but our P.O. box number is five digits. And our +4 is the same as the last 4 digits of our P.O. box number. For a long time I trusted with the clerk had written on the sign-up form and our address was listed everywhere with the +4 being the first 4 digits.

After I checked with usps.com and saw that our +4 was actually the last 4 digits of our P.O. box (I am assuming that usps.com is correct and the clerk was wrong), I corrected our address everwhere I could (website etc.)