An apostrophe is a punctuation mark used primarily to indicate possession or to form contractions. For example, in the phrase Sarah's book, the apostrophe shows that the book belongs to Sarah. In contractions, such as can't (cannot) or it's (it is), the apostrophe replaces omitted letters. Apostrophes are also used in plural forms of letters, symbols, or numbers to avoid confusion. For instance, one might write mind your p's and q's to clarify the letters being referred to. However, apostrophes should not be used for plural nouns, such as apples.