Theirs is a possessive pronoun that shows something belongs to other people. When you say “That backpack is theirs,” you mean it belongs to them, not to you or anyone else.

Unlike other possessive words, theirs doesn't need an apostrophe. You write “theirs,” not “their's.” It also stands alone without a noun after it. You say “the choice is theirs” or “those books are theirs,” but you wouldn't say “those are theirs books.” For that, you'd use their instead: “those are their books.”

The word can refer to things owned by two people or by many people. If your friends brought sandwiches to share, you might point to a cooler and say “Theirs is the blue one.” If another class wins the school science fair, you might say “The trophy is theirs.”

Sometimes theirs appears in formal expressions like “a friend of theirs” (meaning one of their friends) or when dividing things fairly: “We'll take ours to the left field, and they'll take theirs to the right field.”