In deciding whether or not to use, and where to place, an apostrophe after a genitive (or possessive) word, I have always relied on men. That’s Since about 1900, many business firms, institutions, and ...

First, the possessive pronouns "theirs, his, hers, ours, its, yours" have no apostrophes. Second, all other possessive words do have apostrophes, as in "Joe's cat." ...