To form the possessive of a singular noun- James, in this case -where you can hear the s and a new syllable is formed in the pronunciation of the possessive (in other words it would sound like Jameses), add an apostrophe plus s . Therefore, it would be James's cat. However, if the possessive of a singular noun; for example, Illinois, where the s is silent--in other words, you don't pronounce the last s in Illinois--add an apostrophe plus s .
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