There are about ten abbreviations that make the best sense for the Post Office/Postal Service, but not so much sense elsewhere, for various reasons. For example, Nebraska can't be NB because that's also New Brunswick, Arkansas can't be AS because that's American Samoa, and Mississippi can't be MP (P being the only unique letter in Mississippi for states beginning with M) because MP used to stand for "Marianas Protectorate".
In my humble opinion, the most ambiguous, arbitrary or illogical abbreviations — the ones that get confused even on TV news and in charts — are AL (Alaska?), AR (Arizona?), AK (Arkansas?), MA (Maine? Maryland?), MI (Minnesota?, Mississippi?, Missouri?), MS (Massachusetts?), MT (Massachusetts?), MN (Michigan? Montana? Maryland? Maine?), MO (Montana?), NE (Nevada? New England?) and IA (Indiana?) |