I grew up sort of unconsciously assuming that there were fairly straightforward rules, fairly consistently applied, for how to turn a place name into the term for the people who live there. Once I actually looked, it turned out I had not appreciated how complex and inconsistent it is. I think it was during the 2000 election controversy, when people on TV kept talking about “Floridians”, that it sunk in for me that there’s no requirement for similar sounding place names to be consistent: it could just as easily be “Floridan” and “Nevadian” as the other way around.
I think I will now inventory the demonyms for people who live in the US states and territories, according to what rule they empirically seem to have used. And I’ll throw in Canadian provinces and Australian states too.
Global rule to apply before other rules: if the place name is a plural, convert it to singular before looking for a rule to apply below.
Cases following this rule: Mariana Islands, Virgin Islands, Northwest Territories.
First rule: if the place name ends in “ia”, just add “n”.
Cases following this rule: California, Georgia, Pennsylvania, Virginia, West Virginia, British Columbia, Nova Scotia, South Australia, Western Australia, Tazmania, Victoria.
Exception: District of Columbia (people just say “Washingtonian”).
Second rule: if the place name ends in “a” but not in “ia”, just add “n”.
Cases: Alaska, Arizona, Iowa, Minnesota, Montana, Nevada, North and South Dakota, Oklahoma, Samoa, Alberta, Manitoba. (Also, America.)
Exceptions: see next rule.
Third rule: if the place name ends in “a” but not in “ia”, and you don’t want to follow the second rule, you can replace the “a” with “ian”.
Cases: Alabama, Florida, Indiana, Louisiana, North and South Carolina. (And Canada.)
Remaining exceptions: none.
Fourth rule: if the place name ends in an “ee” sound, add “an”.
Cases: Hawaii, Illinois, Mississippi, Missouri, New Jersey, Northwest Territories.
Minor exceptions: Kentucky (the “y” is replaced with “i”), Tennessee (the last “e” is dropped), Northern Territories (just “Territorian” with no “Northern”).
Real exception: Australian Capitol Territory (similarly to D.C., it gets covered by “Canberran”).
Fifth rule: if it ends in a vowel sound not covered above, replace that vowel (and any silent letter following it) with “an”.
Cases: Arkansas, Colorado, New Mexico, Ontario, Puerto Rico.
Exceptions: see next rule.
Sixth rule: if it ends in a vowel sound not covered in the first four rules, and you don’t want to follow the fifth rule, add “an” without removing anything.
Cases: Idaho, Ohio.
Remaining exception: Utah (it’s “Utahn”).
Seventh rule: if it ends with “as”, replace the “s” with “n”.
Cases: Kansas, Texas.
Eighth rule: if it ends in a plosive or unvoiced consonant sound, but not with “as”, add “er”.
Cases: Maryland, Massachusetts, New York, Rhode Island, Vermont, Mariana Islands, Virgin Islands, New Brunswick, Newfoundland, Prince Edward Island, Queensland. (Also, New Zealand.)
Minor exceptions: Connecticutt (the last “t” is dropped), Quebec (any of “Quebecker” or “Quebecer” or “Québécois”).
Real exception: Nunavut (“Nunavummiuq”).
Ninth rule: if it ends with a voiced nonplosive consonant sound, add “ian”, or just “an” if the last letter is a silent “e”.
Cases: Delaware, Oregon, Washington, Labrador, Saskatchewan.
Exceptions: see next rule. Also, Michigan can follow this rule, but it’s optional, as it has two competing official demonyms.
Tenth rule: if it ends with a voiced nonplosive consonant sound, and you don’t want to follow the ninth rule, add “ite”, after dropping any silent “e” at the end.
Cases: New Hampshire, Wisconsin, Wyoming (most people pronounce this nonplosively).
Remaining exceptions: see next rule.
Eleventh rule: if it ends with a voiced nonplosive consonant sound, and you don’t want to follow the ninth rule or the tenth rule, add “er”, or just “r” if the last letter is a silent “e”
Cases: Maine, Yukon.
Remaining exceptions: Michigan (the second official demonym adds “der”), Guam (“Chamorro”), New South Wales (“New South Welshman”).
I’ve restricted this list to English-speaking nations, but I excluded the British Isles themselves: the adjectival forms “English”, “Scottish”, “Irish”, and “Welsh” have their own rule, and for added inconvenience “Englishman”/”Scotsman”/”Irishman”/”Welshman” are gendered. “Briton” doesn’t fit with anything else. And I wouldn’t know where to begin with smaller regions such as Middlesex or Yorkshire or Cork. Given the existence of a case like “Manx”, I don’t even want to look.
And once we get into nonanglophonic areas of the world, anything can happen, even if sticking to Englishized names.