Possessive Form of Singular Nouns Ending with 'S'

Many people struggle with the possessive case of singular nouns when the words already end with 's'. The general rule is this:
Form the possessive singular of nouns with 's'.

Here are some examples:

"James‘s cat"
"Mrs. Jones‘s attorney"
"Dr. Seuss‘s book"

Exceptions:
Of course, we’re talking about the English language, so we’re going to have some exceptions to the rule. While grammar books and style guides don’t necessarily agree on how to determine these exceptions, most consider a word’s pronunciation. Here is what a few of the books say:

- "If pronunciation would be awkward with the added -’s, some writers use only the apostrophe. Either use is acceptable." (Diana Hacker, A Writer’s Reference)
- "Exceptions are the possessives of ancient proper names in -es and -is [such as Achilles' and Isis'], the possessive Jesus’, and such forms as for conscience’ sake, for righteousness’ sake." (Strunk and White, The Elements of Style)
- "With some singular nouns that end in -s, pronouncing the possessive ending as a separate syllable can sound awkward; in such cases, it is acceptable to use just an apostrophe." (Kirszner & Mandell, The Brief Holt Handbook)
- "Since writers vary in the use of the apostrophe, it is not possible to make a hard and fast rule about the apostrophe in singular words ending in s.… Punctuate according to pronunciation." (John E. Warriner, English Grammar and Composition)


http://www.englishrules.com/writing/2005/possessive-form-of-singular-nouns-ending-with-s/