What punctuation mark shows possession in the phrase "Sarah's book"?

The punctuation mark showing possession in “Sarah’s book” is an apostrophe. Here, the apostrophe followed by s marks Sarah as the person associated with or possessing the book.

In English, a singular noun usually forms its possessive with ’s: “the child’s toy,” “the teacher’s desk,” or “Sarah’s book.” The relationship need not mean literal ownership; expressions such as “today’s weather” and “the school’s principal” show association or connection instead.

Plural nouns that already end in s usually take only an apostrophe: “the students’ projects.” Plural nouns that do not end in s generally take ’s, as in “children’s books” and “women’s clothing.” These patterns help distinguish possession from an ordinary plural.

A common mix-up involves its and it’s. “Its” is the possessive form, while “it’s” is a contraction of “it is” or “it has.” Apostrophes also mark omitted letters in contractions such as “don’t,” so possession is one of the mark’s two main uses in English.