possesive as a noun

[kml]

Why is it correct to use the expression "He is a friend of Harry's." instead of "He is a friend of Harry." or "He is Harry's friend."? Convention seems to dictate that the first sentence is correct, even though "Harry's", an object of the preposition "of", is a possessive ?adjective and not a noun. He is a friend of Harry's what? Is this not grammatically incorrect?

[Buddhaheart]

It’s correct to say "He is a friend of Harry's." This is called the double possessive, double genitive, independent genitive or appositional of-phrase. It’s an old and well-established idiom. It’s equivalent to he is one of Harry’s friends. It is indefinite. You won’t say he is a friend of *me or *you, you say he is a friend of mine or yours; why would you say "He is a friend of Harry?" Harry’s friend is fine. You mean a particular one. "Harry's" is a possessive pronoun.

It’s grammatically incorrect or doesn’t make sense to ask "He is a friend of Harry's what?"