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I vaguely recall memorizing the state abbreviations in grades school. And I seem to remember my teacher sharing a few ground rules for helping us memorize them.

  • No two states may have the same abbreviation.
  • If there are two words in the state's name, take the first letter of each. (NY, SD, WV, etc)
  • Take the first two letters of the state name to form the abbreviation.
  • Take the first and last letter of the state name to form the abbreviation.

The first two rules are never broken. The second two are broken as needed in order to obey the first one. But which one takes precidence? What other considerations come into play that would have led to choices such as FL, HI, NV

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Well, FL fits rule 3, and HI fits rule 4. NV doesn't follow these rules though, since "NA" is available. –  Matt Mar 9 '12 at 21:34
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Even more interestingly, the two-letter abbreviations for Canadian provinces are also unique with respect to those of the American states: they form one coherent namespace comprising both countries’ abbreviations. –  tchrist Mar 10 '12 at 4:26
    
That was deliberate: The USPS changed Nebraska's abbreviation from NB to NE to avoid confusion with Canada's New Brunswick. –  Dan Oct 10 '13 at 4:43
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3 Answers

up vote 8 down vote accepted

From this PDF which cites various USPS memoranda, we can see that state abbreviations went through several stages, and most of the "surprising" 2-letter abbreviations (like AK for Alaska, NV for Nevada, etc.) can be explained as first-letter/last-letter abbreviations of already existing abbreviations.

First two letters

+----------------------+-------+
| Alabama              | AL    |
| Arkansas             | AR    |
| California           | CA    |
| Colorado             | CO    |
| Delaware             | DE    |
| Florida              | FL    |
| Idaho                | ID    |
| Illinois             | IL    |
| Indiana              | IN    |
| Massachusetts        | MA    |
| Michigan             | MI    |
| Nebraska             | NE    |
| Ohio                 | OH    |
| Oklahoma             | OK    |
| Oregon               | OR    |
| Utah                 | UT    |
| Washington           | WA    |
| Wisconsin            | WI    |
| Wyoming              | WY    |
+----------------------+-------+

First and last letter

+----------------------+-------+
| Connecticut          | CT    |
| Georgia              | GA    |
| Hawaii               | HI    |
| Iowa                 | IA    |
| Kentucky             | KY    |
| Louisiana            | LA    |
| Maine                | ME    |
| Maryland             | MD    |
| Pennsylvania         | PA    |
| Vermont              | VT    |
| Virginia             | VA    |
+----------------------+-------+

Initial letter of 2 words

+----------------------+-------+
| District of Columbia | DC    |
| New Hampshire        | NH    |
| New Jersey           | NJ    |
| New Mexico           | NM    |
| New York             | NY    |
| North Carolina       | NC    |
| North Dakota         | ND    |
| Puerto Rico          | PR    |
| Rhode Island         | RI    |
| South Carolina       | SC    |
| South Dakota         | SD    |
| West Virginia        | WV    |
+----------------------+-------+

From first and last letter of old abbreviation

+-------------+------+-------+
| Alaska      | ALSK | AK    |
| Arizona     | ARIZ | AZ    |
| Kansas      | KANS | KS    |
| Minnesota   | MINN | MN    |
| Mississippi | MISS | MS    |
| Montana     | MONT | MT    |
| Nevada      | NEV  | NV    |
| Tennessee   | TENN | TN    |
| Texas       | TEX  | TX    |
+-------------+------+-------+

Note that Missouri's abbreviation MO appears to be a historical anomaly dating back to at least 1831, deriving from the fact that both it and Mississippi start with MISS and end in I. O is the first distinct letter to Missouri's spelling, and Mississippi presumably got precedence for MS because it achieved statehood in 1817 whereas Missouri became a state in 1821. It's unclear why Michigan had precedence over both of them with MI, as it became a state after both (1837).

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How interesting that most of the "exceptions" are given by the first and last letter of old abbreviation. Where did you find those old abbreviations? –  Stainsor Mar 26 '12 at 16:04
    
@Stainsor I found them in the PDF linked at the top of the answer. –  nohat Mar 26 '12 at 18:30
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The main problem is that no simple convention will work without creating duplicates.

Put yourself in the shoes of the Postmaster General, when you want to start standardizing two-letter abbreviations for each state. One of your unenviable tasks revolves around the fact that 8 states out of 50 begin with the same letter (M). None of these have two-word state names, so what can you do?

You can try using the first two letters, but 7 of the 8 states begin with "Ma" or "Mi". To break the ties, so to speak, you can try using the first and last letters instead:

Maine
Maryland
Massachusetts

Only one can get MA, so the other two get ME and MD. (We don't want to give MS to Massachusetts - not with Mississippi and Missouri still needing abbreviations).

Michigan
Minnesota
Mississippi
Missouri

Only one of these four can get MI, so the other three get... hmmmm, now we're stuck. We can't use the last letter: MI and MA are already taken. We could give MN to Michigan, but then what would be used for Minnesota? Every other letter in Minnesota is used by another state - this gets as hard as a Sudoku puzzle!

Montana

I've always figured that, given how the M_ abbreviations are at such a premium, it makes sense to give MT to Montana, because "Mt" is an abbreviation for "mountain," and "mountain" and "Montana" have the same root.

After the M puzzle gets solved, there aren't as many conflicts. Most often, either the first two letters get used, or else the first and last letters are used. In some cases, either algorithm would give the same result (California, Colorado, Delaware). Other times, one answer simply seems more intuitive than the other (Ohio as OH instead of OO, Idaho as ID instead of IO, e.g.). Because there are far more consonants than vowels, though, sometimes a prominent consonant is used instead of the second or last letter, particularly when following the 2nd- or last-letter algorithm would produce a more confusing result (Nevada as NV instead of NE or NA - which could be easily confused with Nebraska; Texas as TX instead of TE or TS - which could be easily confused with Tennessee).

A few of the remaining dilemmas seem rather arbitrary: Hawaii could have been HA, HW, or HI, and still followed convention.

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Many newspapers and publishing companies follow the Associated Press Stylebook, and the AP style does not use the two-letter postal abbreviations for states.

This article has a chart that includes the AP style abbreviations List of U.S. state abbreviations

In AP style, you use abbreviations for most state names when following a city name (for example, "Scranton, Pa."). In AP style, you always spell out Alaska, Hawaii, and state names with five or fewer letters.

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