(1) Singular nouns, and plural nouns not ending in s, make the possessive form by the addition of the apostrophe and s; thus, dog's, man's, men's.

(2) Plural nouns ending in s make the possessive form by the addition of the apostrophe only; thus, dogs', boys'.

Exception

Usage differs in regard to the possessive form of singular nouns ending in s, sh, soft ch, x, and z. Some careful writers add the apostrophe only to such words if they contain more than one syllable; thus, goodness? sake, Socrates1 wisdom. But the tendency seems to be to pronounce and write the s unless doing so makes a word very difficult to pronounce. An extra syllable may be made if necessary. The possessive of proper names of one syllable should always have both the apostrophe and s; thus, Robert Burns's poems, Keats's life, James's hat.