That's something I'd like to ask you US folks: I used to believe that the names of states were always written with a capital letter to start them and then followed with a lower case, as above. Recently, I've noticed them written as LA, FL (Fla?) etc. so I wondered which way was correct.
Rob C
Some years ago the U.S. Postal Service decreed a set of "official" state name abbreviations for use in postal addresses. Those are the two-letter ones like AZ, MA, LA, NV, etc. Both letters are supposed to be capitals. Long before these "official" abbreviations came on the scene, lots of other abbreviations were in common use, such as Ariz, Ala, Miss, Mass, Cal or Calif, etc. States with two-word names generally had two-letter abbreviations, both capital, such as NH for New Hampshire and SC for South Carolina. I haven't checked, but I believe all the traditional two-capital-letter abbreviations were kept as part of the "official" set.
Now you know more than you ever wanted to know about U.S. state name abbreviations. Oh, by the way, U.S. is correct and AZ is correct, but A.Z. is not.
Cheers,
Eric (from MA, Mass, or Mass.)