# Why use Zip+4 numbers for P.O. boxes? **URL:** \ **Category:** Factual Questions\ **Created:** [February 28, 2009, 3:20am UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755 "2009-02-28T03:20:19Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Elendil\_s\_Heir](https://avatars.discourse-cdn.com/v4/letter/e/7cd45c/32.png) [@Elendil\_s\_Heir](https://boards.straightdope.com/u/Elendil_s_Heir)\ **Post date:** [February 28, 2009, 3:20am UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/1 "2009-02-28T03:20:19Z")
Every time I’ve seen it done, it just repeats the last four digits of the P.O. box anyway. What’s the point? ---
**Author:** ![RealityChuck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/realitychuck/32/195_2.png) [@RealityChuck](https://boards.straightdope.com/u/RealityChuck)\ **Post date:** [February 28, 2009, 3:39am UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/2 "2009-02-28T03:39:54Z")
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 \ Podunk, NT, 02345-0128 It gets delivered to box 128. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [February 28, 2009, 3:49am UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/3 "2009-02-28T03:49:46Z")
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. ---
**Author:** ![bibliophage](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bibliophage/32/7615_2.png) [@bibliophage](https://boards.straightdope.com/u/bibliophage)\ **Post date:** [February 28, 2009, 7:30am UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/4 "2009-02-28T07:30:16Z")
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. ---
**Author:** ![clayton\_e](https://avatars.discourse-cdn.com/v4/letter/c/c57346/32.png) [@clayton\_e](https://boards.straightdope.com/u/clayton_e)\ **Post date:** [February 28, 2009, 9:18am UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/5 "2009-02-28T09:18:29Z")
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? ---
**Author:** ![Mudshark](https://avatars.discourse-cdn.com/v4/letter/m/f04885/32.png) [@Mudshark](https://boards.straightdope.com/u/Mudshark)\ **Post date:** [February 28, 2009, 11:37am UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/6 "2009-02-28T11:37:18Z")
I was under the impression the last four were for the individual house/building. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [February 28, 2009, 2:54pm UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/7 "2009-02-28T14:54:00Z")
> [@clayton\_e](#): > > 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? 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. ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [February 28, 2009, 5:57pm UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/8 "2009-02-28T17:57:00Z")
> [@Mudshark](#): > > I was under the impression the last four were for the individual house/building. 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. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [February 28, 2009, 6:21pm UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/9 "2009-02-28T18:21:49Z")
> [@Polycarp](#): > > 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. 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. ---
**Author:** ![dbygawdcapn](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dbygawdcapn/32/3995_2.png) [@dbygawdcapn](https://boards.straightdope.com/u/dbygawdcapn)\ **Post date:** [February 28, 2009, 6:37pm UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/10 "2009-02-28T18:37:13Z")
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. ---
**Author:** ![clayton\_e](https://avatars.discourse-cdn.com/v4/letter/c/c57346/32.png) [@clayton\_e](https://boards.straightdope.com/u/clayton_e)\ **Post date:** [March 1, 2009, 4:36am UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/11 "2009-03-01T04:36:18Z")
> [@Polycarp](#): > > Substitute mail carriers. Doesn’t your carrier ever have a day off or go on vacation? 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.. ---
**Author:** ![Keeve](https://avatars.discourse-cdn.com/v4/letter/k/f07891/32.png) [@Keeve](https://boards.straightdope.com/u/Keeve)\ **Post date:** [March 1, 2009, 11:44am UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/12 "2009-03-01T11:44:47Z")
I’ve seen many cases where the first of the four digits differs. ---
**Author:** ![Rayne\_Man](https://avatars.discourse-cdn.com/v4/letter/r/df705f/32.png) [@Rayne\_Man](https://boards.straightdope.com/u/Rayne_Man)\ **Post date:** [March 1, 2009, 12:29pm UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/13 "2009-03-01T12:29:57Z")
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. ---
**Author:** ![Arnold\_Winkelried](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@Arnold\_Winkelried](https://boards.straightdope.com/u/Arnold_Winkelried)\ **Post date:** [March 1, 2009, 1:37pm UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/14 "2009-03-01T13:37:28Z")
> [@Elendil\_s\_Heir](#): > > Every time I’ve seen it done, it just repeats the last four digits of the P.O. box anyway. [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](http://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. ---
**Author:** ![St.Urho](https://avatars.discourse-cdn.com/v4/letter/s/8e7dd6/32.png) [@St.Urho](https://boards.straightdope.com/u/St.Urho)\ **Post date:** [March 1, 2009, 1:39pm UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/15 "2009-03-01T13:39:41Z")
> [@Elendil\_s\_Heir](#): > > Every time I’ve seen it done, it just repeats the last four digits of the P.O. box anyway. What’s the point? The +4 for my P.O. Box is not the same as the last 4 digits of the box number. ---
**Author:** ![tetranz](https://avatars.discourse-cdn.com/v4/letter/t/a587f6/32.png) [@tetranz](https://boards.straightdope.com/u/tetranz)\ **Post date:** [March 1, 2009, 6:10pm UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/16 "2009-03-01T18:10:46Z")
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? ---
**Author:** ![Arnold\_Winkelried](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@Arnold\_Winkelried](https://boards.straightdope.com/u/Arnold_Winkelried)\ **Post date:** [March 2, 2009, 9:56pm UTC](https://boards.straightdope.com/t/why-use-zip-4-numbers-for-p-o-boxes/487755/17 "2009-03-02T21:56:14Z")
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](http://usps.com) and saw that our +4 was actually the last 4 digits of our P.O. box (I am assuming that [usps.com](http://usps.com) is correct and the clerk was wrong), I corrected our address everwhere I could (website etc.)