Possessive Pronouns Never Take an Apostrophe

Regular nouns need an apostrophe to show ownership: the team's mascot, the architect's drawings. Pronouns work differently. English built a separate set of possessive forms for pronouns a long time ago: mine, yours, his, hers, its, ours, theirs, whose. Ownership is already baked into the word itself, so there's no job left for an apostrophe to do. Adding one doesn't add meaning; it just breaks the spelling of a word that already means what you want.

This is why 'their's' and 'her's' aren't real words at all, even though 'the team's' and 'the dog's' are correct right next to them. The noun needs the apostrophe to borrow possession; the pronoun already owns it. The same logic explains its versus it's, and whose versus who's. Its and whose are possessive. It's and who's are always contractions: it's means 'it is' or 'it has,' and who's means 'who is' or 'who has.' If you can't expand the word into 'is' or 'has' and have the sentence still make sense, the apostrophe doesn't belong there.

This sits inside the bigger punctuation question of what an apostrophe is actually for: joining two words into one (contraction) or marking possession on a noun. Possessive pronouns fall outside both jobs in their usual spelling, which is exactly why they break the pattern. Spotting a possessive pronoun standing alone, as in 'The red one is hers' or 'The choice was theirs,' confirms the same rule: no apostrophe, no noun attached, ownership is simply the word's meaning.