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Possessives and Attributives
Q. When indicating possession of a word that ends in s, is it correct to repeat the s after using an apostrophe? For example, which is correct: “Dickens’ novel” or “Dickens’s novel”?
A. Either is correct, though we prefer the latter. Please consult CMOS 7.16–19 for a full discussion of the rules for forming the possessive of proper nouns. For a discussion of the alternative practice of simply adding an apostrophe to form the possessive of proper nouns ending in s, see paragraph 7.22.