CMOS 7.16 Possessive of proper nouns, letters, and numbers
“The general rule extends to proper nouns, including names ending in s, x, or z, in both their singular and plural forms, as well as letters and numbers.
the Lincolns’ marriage
1999’s heaviest snowstorm”
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The general rule referenced by this section comes from 7.15. You’re likely already familiar with the basics: “The possessive of most singular nouns is formed by adding an apostrophe and an s. The possessive of plural nouns (except for a few irregular plurals, like children, that do not end in s) is formed by adding an apostrophe only.” My dog’s fleas; the boys’ books; the children’s school.
The question is what to do with proper names that are already possessive, like Kinko’s or Sotheby’s. Readers wonder whether to add a second apostrophe and s in phrases like “Kinko’s’s current publicity campaign.”
The answer is no. If rephrasing is for some reason forbidden (“the publicity campaign that Kinko’s is now running”), just leave the original possessive as is: “Kinko’s current publicity campaign.”
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