The Purdue writing guide gives this rule: add 's to the singular form of the word (even if it ends in -s): the owner's car James's hat (James' hat is also acceptable. htm Some writers will say that the -s after Charles' is not necessary and that adding only the apostrophe (Charles' car) will suffice to show possession. Consistency is the key here: if you choose not to add the -s after a noun that already ends in s, do so consistently throughout your text.
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