Possessive pronouns describe what things belong to which people, like 'her shoe' or 'the book is mine.' Possessive pronouns can be adjectives, like 'his bicycle,' or they can stand in for nouns, like 'the seats are theirs.' Neither of these forms should have apostrophes to show possession -- so it's ours (not our's) and yours (not your's).

Common Core

L.6.1.a