Illinoises or Illinois’s or Illinois’? (Correct Possessive Explained)

The possessive form comes with a few different rules, and we need to know what they are. When using proper nouns, like “Illinois,” we have to make sure we understand the two separate rules that might come with it. This article will help you understand those rules.

Illinoises or Illinois’s or Illinois’: Which Is The Correct Possessive Form?

“Illinois’s” and “Illinois'” are correct possessive forms of the state “Illinois.” We use “Illinois’s” according to the Microsoft and Chicago Manuals of Style, which is the most commonly written style. We use “Illinois'” according to the Associated Press Stylebook guidelines.

Illinoises or Illinois's or Illinois': Which Is The Correct Possessive Form?

It might help you to look at the following to help you understand the difference between the singular form and the possessive form.

  • Singular: Illinois
  • Singular Possessive: Illinois’s or Illinois’

There is no plural of Illinois since it’s only one state in the US. You can’t refer to more than one “Illinois” in a sentence (unless two places named “Illinois” existed). Therefore, we don’t worry about the plural form or the plural possessive form.


The first of the forms we want to run you through is incorrect. Why are we starting with it if it’s incorrect? Well, we’re trying to help you understand why you shouldn’t use it.

“Illinoises” would imply that there is a plural form of “Illinois.” It is incorrect because there’s no such case where we would talk about more than one “Illinois.” We would add the “-es” ending to words that are capable of being pluralized.

For this reason, there is no time where “Illinoises” works. Instead, you’ll only want to stick to using the singular form since it’s much more likely that you’ll be talking about “Illinois” as a singular entity.

  • Correct: Where can I find Illinois on the map?
  • Incorrect: How many Illinoises are there in America?
  • Correct: I live in Illinois, and I love it there!
  • Incorrect: We should find where Illinoises are located.
  • Correct: Illinois is probably my favorite state!
  • Incorrect: I love Chicago, which is a city in Illinoises.


Now that we’ve got the problematic one out of the way, it’s time to look at the two possible forms that are correct. The possessive form for proper nouns has two apparent rules, and we can use both depending on our writing.

“Illinois’s” is correct when following the Microsoft Manual of Style or the Chicago Manual of Style. Both styles teach us that any name or noun that ends in an “S” still requires the “‘s” ending after the word to indicate the possessive form.

We use the double “S” ending to help with reading comprehension. It’s much easier to notice the possessive form when we can see an “S” on either side of the apostrophe.

Some examples will help you to understand how “Illinois’s” works:

  1. Illinois’s citizens are not exactly happy with the new laws we’ve passed.
  2. Illinois’s governors are having a hard time convincing people that they know what they’re talking about.
  3. We are not Illinois’s only hope, and there are plenty of others out there who can do what we can do.
  4. Illinois’s main export is a mystery to me. I don’t know why they’re so rich.
  5. Illinois’s most famous city is definitely Chicago!
  6. Which of Illinois’s major landmarks do you appreciate the most?

“Illinois’s” is one of the two possible possessive forms we can use. It works when we treat “Illinois” as the sentence’s subject. The “‘s” ending shows that “Illinois” owns an object in the sentence, which is the word or group of words that directly follows the noun.


“Illinois'” follows much the same rules as “Illinois’s.” The only reason we might use it instead is based on the style we write with.

“Illinois'” is correct when following the Associated Press Stylebook guidelines. According to AP rules, all names and nouns that end in an “S” do not need the extra “S” after the apostrophe as it is a redundant letter.

The possessive form still works in the same way regardless of the extra “S.” It’s believed that dropping the “S” after the apostrophe helps writers to keep their writing uniform and streamlined while allowing readers an easier time to pronounce certain words.

AP rules are commonly taught in schools, though they’re not the most common writing style outside of education.

Some examples will show you how it looks:

  1. Illinois’ people are getting progressively angrier at you!
  2. Illinois’ night skyline is beautiful when looking at it from over here.
  3. We are Illinois’ most adored citizens, and we won’t let you stop us from having fun.
  4. Where is Illinois’ most famous attraction?
  5. Illinois’ movie studios have created some of the most famous stars known to the world.
  6. We are part of Illinois’ congregation of lovable people!

There are no rules that tell us that we can use “Illinois’s” or “Illinois'” over each other. Since they are both correct, it comes down to a matter of style. The only thing you have to remember is that, whichever style you pick, you must keep it the same throughout your writing.

Quiz: Have You Mastered Illinoises or Illinois’s or Illinois’?

A quiz will help you to understand a little more about all the forms present here. If you’ve been paying attention, you should have an easy time with this one!

  1. (A. Illinoises / B. Illinois’s / C. Illinois’) track record in this regard isn’t the cleanest.
  2. We need to find (A. Illinoises / B. Illinois’s / C. Illinois’) citizens before it’s too late.
  3. (A. Illinoises / B. Illinois’s / C. Illinois’) architecture is some of the most remarkable work I’ve ever seen.
  4. (A. Illinoises / B. Illinois’s / C. Illinois’) talent is unlike anything else I’ve seen in the world.
  5. (A. Illinoises / B. Illinois’s / C. Illinois’) inhabitants really make you question your own livelihood!

Quiz answers

  1. B or C
  2. B or C
  3. B or C
  4. B or C
  5. B or C

Both B and C are correct, but you should only ever stick to the same answer. If you use “Illinois’s” once, use it always in your writing. If you use “Illinois'” instead, make sure you stick to that.

Final Thoughts

“Illinois’s” and “Illinois'” are both correct depending on the writing style you choose to adopt. No rules tell you how to use one over the other. The only thing you have to remember is to stick to one style throughout your writing, so you don’t switch between the two.

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