You know what a double genitive is, even if you don’t realize it. “He’s a friend of Maud’s” is an example of a double genitive. Are you a “friend of Paul,” or a “friend of Paul’s”? Are you a “cousin of Jamal,” or a “cousin of Jamal’s”?
- I remember that guy; he is an acquaintance of my uncle.
- Hey, that mug is one of my roommate.
- He is a colleague of Ruth.
He is an acquaintance of my uncle’s, not of mine. My roommate has many mugs, and that mug is his — my roommate’s. There is some subtlety here: The apostrophe places the focus on the person. “A colleague of Ruth’s” indicates that Ruth is the one who considers him the colleague. He is a colleague of hers; he is a colleague of Ruth’s.
MORE TO KNOW
This grammatical construction is oddly controversial, but it needn’t be. A double genitive indicates possession by the preposition of followed by the possessive form of a noun, pronoun, or name. As soon as you use of, as in many of, the of indicates possession: of mine, one of his, many of the school’s. Adding the apostrophe plus the letter s indicates that second level of possession. It is an idiomatic construction, and it is correct. Substitute a possessive pronoun (mine, yours, his, hers, its, ours, yours, and theirs) for the name and the apostrophe plus the letter s, and you’ll still be right.