Words of one syllable follow the first rule: "James's share." Some words of two syllables follow the first rule, some the second: "the princess's birthday"; "Francis' style."
This distinction is sanctioned by usage. But it may judiciously be disregarded. In speaking we almost entirely ignore it. Why should we trouble ourselves with it in writing?
Nouns in the plural number—
(1) The apostrophe is placed after the "s" of the plural.
Boys' clothing. Our friends' troubles.
(2) If the plural do not end in "s," an "s" is added, and the apostrophe is placed before it.
Men's opinions. The children's pleasure.
LXXVI. The apostrophe is used before the "s" of the plural when single letters are used as words.
Mind your p's and q's.