The punctuation mark used to indicate possession, as in “John’s book,” is an apostrophe. In this construction, the apostrophe marks the possessive form of the singular name “John.”
For most singular nouns, English adds apostrophe plus s: “the cat’s whiskers,” “the school’s headmaster,” or “tomorrow’s weather.” The relationship need not mean literal ownership; it can indicate association, origin, or connection. A school does not physically possess its headmaster, for example.
Plural nouns that already end in s usually take only an apostrophe: “the students’ desks.” Irregular plurals without a final s take apostrophe plus s, as in “children’s books” and “men’s clothing.” A frequent mix-up is “its” versus “it’s”: “its” is possessive, while “it’s” is a contraction of “it is” or “it has.” The apostrophe also marks omitted letters in contractions such as “do not” becoming “don’t.”