Theirs or their's: which one is correct?

Theirs is correct; their's does not exist. Possessive pronouns such as theirs, ours, hers, and yours never take an apostrophe, unlike nouns such as Anthony's.

Learning English is especially hard for non-native speakers when it comes to homophones: words that sound the same but differ in meaning, and sometimes in spelling too. You can learn more about homophones and how they differ from homonyms and homographs.

Many groups of words that sound alike are easy to confuse, for example there, their, and they're or where, were, wear, and we're. This article looks at another pair: theirs and their's. (Theirs also sounds just like there's, short for "there is", which adds to the confusion.)

What is the difference between theirs and their's?

To see the difference between the two, look at what each one means.

Theirs

Theirs is a possessive pronoun in the third person plural. In a sentence, it replaces their together with the thing that is owned, a noun. Compare these two examples:

1. I found some keys on the pavement. They could be Carla and Ben's: they lost theirs.
2. I found some keys on the pavement. They could be Carla and Ben's: they lost their keys.

In the first example, theirs replaces the combination of their and a noun in the second example: their keys.

Their's

The case of their's is simple: it does not exist. Even native speakers of English often make this mistake, but their's with an apostrophe is never a correct way to show possession.

Why is that so?

The mistake is easy to understand. In almost every other word, an apostrophe and an s show possession:

• The dog's bone.
• Anthony's grades.
• My mother's new car.

Pronouns are different, though. As with its and it's, a possessive pronoun shows possession without an apostrophe. These pronouns work exactly the same way:

• I found Maria's pen. At least I think it's hers.
• Martin didn't like my idea, and quite frankly, I think yours is better anyway.
• It must be the Smiths' dog that is barking, because ours is at the vet.

Conclusion

Possessive pronouns never take an apostrophe. When one ends in the letter s, such as hers or theirs, the s is part of the word itself.

A spell checker such as the Spell Checker compares each word with its dictionaries, which helps you catch wrong forms like these before anyone else sees them.