Marie asks a good question:
Could you please address the possessive of Illinois? I've grown tired of writing, "the people of Illinois..." Is it, "Illinois's people...?" Or, "Illinois' people..."
Also, could you address how the possessive is spoken. Would it sound like, "Illinoises" or "Illinois?"
Forming the possessive of a noun ending in S or Z can be awkward. To answer Marie's question before going on to other issues, I would argue that since the S in Illinois is silent, "Illinois' " doesn't work. So I'd write "Illinois's" and pronounce it "Illinoise."
Some would protest, saying that "Illinoise" is also a dictionary-accepted pronunciation of "Illinois." Fine—in that case, the possessive should be "Illinois's" and pronounced "Illinoises."
Here in Canada we have a lot of French names ending in a silent S or X, and the Globe and Mail's style guide decrees that their possessives shall end in 'S:
Delacroix's (DEL-a-kwa's) paintings
Marois's (Mar-WA's) campaign
Duplessis's (du-ples-SEE's) cabinet
The general rule for possessives of singular nouns ending in a pronounced S, X or Z is to add 'S as with any other singular noun:
But we make a big exception for ancient names ending in an S or Z sound:
This exception seems to reflect a widespread feeling that it sounds silly to say "Pericleezes," "Socrateezes," and "Jesuses." So we'll treat those ancient singular nouns as if they were plurals.
Two more points: First, if we need to turn an S or Z noun into a plural, the possessive form of the plural will be ES':
The Joneses' house
The Dickenses' marriage
The Rodriguezes' business
Second, a plural noun that doesn't end in S takes 'S as its possessive:
And if Marie decides to go back to saying "the people of Illinois," I won't blame her!