ZIP+4

I was mentioning to my brother about having difficulty that we always seem to have some mail that does not include our "apartment" number - which means that the folks at our mail forwarding service must go through an additional step to determine which "apartment" (or folder number) Patti or Dwayne Cartwright are assigned to at 151 Rainbow Dr. When we updated our addresses, we always gave our full address: 151 Rainbow Dr 5103 -- but some of them don't seem to have stored it. It is puzzling to me why this is the case, and each time we get mail, I have 2-3 folks to contact to try, once again, to get them to update our address.

When talking with him, it occurred to me to wonder if they don't include the apartment number because the "ZIP+4" is supposed to contain that information... you know, the additional 4 digits after the 5 digit zip code. I had thought that the "+4" identified down to the house level so, theoretically, the other information is not important. We got to talking about whether 4 additional digits would be sufficient to specifically identify every house in the US, and both of us were going to do more research.

I used Wikipedia and found:

ZIP+4

In 1983, the U.S. Postal Service introduced an expanded ZIP Code system that it called ZIP+4, often called "plus-four codes", "add-on codes", or "add ons". 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. But initial attempts to promote universal use of the new format met with public resistance, and today the plus-four code is not required. In general, mail is read by a multiline optical character reader (MLOCR) that almost instantly determines the correct ZIP+4 Code from the address—along with the even more specific delivery point—and sprays an Intelligent Mail barcode (IM) on the face of the mail piece that corresponds to 11 digits—nine for the ZIP+4 Code and two for the delivery point.

<<< this next paragraph is the part that made me wonder if we had the right ZIP+4 for our address that it would tell which "apartment" or folder the letter should go to... so if we made sure that the sender had the ZIP+4 then it would help the mail forwarding service folks >>>

For Post Office Boxes, the general (but not invariable) rule is that each box has its own ZIP+4 code. The add-on code is often one of the following: the last four digits of the box number (e.g., PO Box 107050, Albany, NY 12201-7050), zero plus the last three digits of the box number (e.g., PO Box 17727, Eagle River, AK 99577-0727), or, if the box number consists of fewer than four digits, enough zeros are attached to the front of the box number to produce a four-digit number (e.g., PO Box 77, Juneau, AK 99750-0077). However, there is no uniform rule, so the ZIP+4 Code must be looked up individually for each box.

So.. I was incorrect that the ZIP+4 would get a piece of mail to a specific address in all cases. But it still may be the case that if I get the right ZIP+4 information on the addresses of mail sent to us, it may help our mail forwarding service folks... at least, I can give it a try! By the way... while our "apartment" number is 5103 (which is like a PO box within the mail forwarding service system), our +4 is 1051... so it doesn't follow the PO box "rule" that Wikipedia had documented.