Canadian province codes are compatible (i.e. non-overlapping) with US state codes: AB, BC, YK, NT, NU, SK, MB, ON, QC, NB, NS, PE, and NL. Former codes, no longer used, include NF, PQ, and LB.
I believe that Mexican state codes may overlap with US state codes (and presumably Canadian province codes).
That’s for lowering characters (M[sub]2[/sub]). Elevating them is {sup}.
Here’s a list. There are quite a few overlaps with the US and Canada, including BC, CO, MI and MO.
That’s a lot less likely than any other additional state, considering it might require amending the Constitution.
I’d guess there are two relevant answers to this–when the two letter codes were officially introduced, and when their use became practically mandatory.
I was born in 1971. The two letter codes have been around as long as I can remember, but when I was a kid, we generally sent things to relatives in Fla. and Conn. Somewhere along the way, I got the idea that we were now “supposed to” use FL and CT. Was there some sort of publicity campaign? How did they get people to make the switch?
Could it have something to do with the advent of optical character recognition by the post office? As I understand it, letters are first scanned for a bar code. If they have no bar code, they are scanned by an OCR system which works remarkably well even on cursive writing. Only those that can’t be deciphered by the OCR get read and sorted by a human. Two letter codes are probably much easier for the OCR to deal with.
I don’t have a cite for the above. It is recalled from an article that I read a couple of years ago. [Useful hints for getting your letters to their destinations faster–use a bar code if possible (MS Word will generate one for you). Use all proper abbreviations. Print the address in block letters.]
My vote’s for Washington 2: Electric Boogaloo.
Well, I think most folks around here who care about the statehood issue are pretty insistent that DC just become a state, no other changes. There’s no real talk about changing its name. I’d imagine that most Washingtonians would prefer to stick with “Washington, DC.”
But I cannot exclude the possibility that, since the pipe-dream of statehood runs through the halls of Congress, the final result might be: Washington, District of Reagan. 
This would go along with the tradition that a territorial and political entity doesn’t necessarily need to be called “State of…” to be a State in the constitutional sense, as the various Commonwealths illustrate. So indeed: Why not stick with District of Columbia?
They were introduced in 1963, at the same time as the 5 digit zip codes. Cite
I’d assume if they made Washington DC a state it’d probably just change from the District of Columbia to the State of Columbia. There’d probably be groups pushing for a new name though, like Reaganland or maybe the Great State of Oprah.
According to here, they were first devised in 1963, at the same time the five-digit ZIP code was introduced.
Speaking from my own observatons and memories, I’d say that the near-universal use of U.S. state abbreviations and ZIP codes only came to pass in the late 70s. When I was a kid, around that time, the Post Office was still sending friendly, encouraging reminders to use the ZIP code, if you would please. Please use the ZIP code? Pretty please? Would it kill you to use a ZIP code, for cripes sake? I mean we went to all this trouble.
Occasionally it causes confusion, especially with all those M and N states. For example, I was watching CNN a couple years ago. At the time, a hurricane was threatening to make landfall on the Gulf Coast. (Gosh, it’s too bad we forgot about that.) The text crawl at the bottom of the screen summed up the situation: “… hurricane Whatsit threatening coasts of TX, LA, MI …”.
See if you can spot the amusing error in that report.
In Germany, you might not have the problem of duplicate city names. (Then again you might; I’d be curious to know.) In the U.S. however, you can have a Springfield, Illinois, as well as a Springfield, Missouri. So the state name — either in full, or as an abbreviation — has practically been required on U.S. mail, probably since the republic began. Long before 1963 anyway. The two-letter abbreviations were just a more efficient, automation-friendly form of what Americans were already using anyway, out of ingrained habit. You always include the state, even for cities and towns in your same state, and even for famous uniquely-named cities like Chicago. Your mail might get delivered just fine if you omitted the state, but why risk it?
Funnily enough, computers have made the two-letter state codes completely obsolete, at least in the realm of delivering mail, which is all that they were for originally. Yet we still seem to be stuck on them. I wouldn’t mind if we switched back to the longer abbreviations. Certainly I believe CNN ought to switch back.
As long ago as 1963? Susan’s address certainly included a Zip Code (which we in Australia hadn’t introduced) when I was writing to her in 1964 but she hadn’t made the transition to CA at that time. I assumed it happened a few years later.
There are a few, the most notable example probably being Frankfurt: There’s the (larger) one in Hesse, Germany’s economic capital with the headquarters of the banks and the largest airport; and one in Brandenburg in the East, right at the Polish border. They’re distinguished by adding the names of their respective rivers, Main (Hesse) and Oder (Brandenburg). Everybody would know what you meant if you referred to “Frankfurt, Hesse,” but it’s just not common.
A few other examples come to my mind, but most of them are smaller cities not very well known abroad (somebody here who knows Freiburg?). Many of them have “descriptive” names which, I think, have been given to the cities during the middle ages when most of Germany’s existing cities were founded.
Oh, and regarding the MI state: I went through Missouri, Mississippi, and Minnesota to find out MI was Michigan (curiously, the only costal state in this list is Mississippi, which was probably what the CNN person had in mind.)
Actually, ZIP codes made them redundant (city names, too). However, it’s good to have that redundancy in addresses, just in case there’s an error in the ZIP code.
This actually gets a lot of discussion here in the DC area. The popular choice for a new state name is New Columbia. And the likely abbreviation? DC.