For proper names like James, AP says, add an apostrophe only: He borrowed James’ car. For generics like boss, add an apostrophe plus S: He borrowed the boss’s car.

Which is correct James’s or James?

James’ birthday, or James’s. The proper convention is to include the possessive apostrophe even when the word ends in an “s.” So “James’s” is correct. The only exception to that are proper nouns so well established that traditionally they have always been used with just an apostrophe.

Where does the apostrophe go in James’s?

It should be St James’s as the apostrophe without the s is used when it is something belonging to a plural. e.g. Our employees’ rights or where the surname is also plural. e.g. Mr Williams’ (the surname is Williams as in a multiple of William).

For names ending in s, form the possessive either by simply adding an apostrophe (James’ books) or by adding an apostrophe as well as another s (Charles’s phone). The possessive of a plural name is always formed by adding an apostrophe after the final s (the Smiths’ dog, the Harrises’ family home).