Think of the simple phrase "Bill's friend".

If you were going to turn this around using the preposition 'of' would you say:

  • A friend of Bill's or
  • A friend of Bill

It appears to me that, in the US anyway, people always say "A friend of Bill's".

Even though I'm a native American English speaker, this just sounds weird to me. It seems to create a 'double possessive' (a term I just invented). I always want to respond: "A friend of Bill's what?" A friend of Bill's aunt?

So what's at work here, and do British English speakers also do this?

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    Good question. Like the "A friend of Bill's what?". Of course, I'm not sure there is an answer... quite a lot of language is "because it has evolved that way". Language is weird. – Jürgen A. Erhard Feb 10 '11 at 20:59

As @FX_ points out, it’s called a double genitive or double possessive.

In this example, it’s not compulsory: both a friend of Bill’s and a friend of Bill are correct, although the first is probably more common. (Usage data, anyone?)

If Bill were replaced by a pronoun, however (poor Bill!), the double genitive would be required: a friend of mine is correct, but not a friend of me. (Similarly with yours vs. you, his vs. he, etc.)

Also, sometimes, this is needed to avoid ambiguity between the possessive and other uses of of: for instance, a picture of Bill’s means that he owns the picture, whereas a picture of Bill means he’s portrayed in it.

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    Replacing Bill with a pronoun really does provide some clarity. I also had not considered 'a picture of Bill's' vs. 'a picture of Bill'. In that case the need for a double possessive is really clear. Well answered! – adj7388 Feb 12 '11 at 12:27
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    There is a huge difference between "a picture of his" and "a picture of him". – pyon Apr 4 '11 at 1:10
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    In the Wikipedia link there is no mention of "double genitive", I reckon the page has since been updated. – Mari-Lou A Nov 23 '13 at 19:28

The correct form is “a friend of Bill’s”, and it's called a double genitive (see also here).

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    Interestingly, Merriam-Webster says it's also called "double possessive". :D – Jürgen A. Erhard Feb 10 '11 at 21:47
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    You are correct that this is a double genitive, but is the double genitive form really the only one that can be considered correct? – Kosmonaut Feb 10 '11 at 21:49
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    I guess I'm not as smart as I thought. I should have googled 'double possessive' before I claimed to have invented it :) – adj7388 Feb 10 '11 at 22:33
  • I think it's too early to anoint "A friend of Bill's" as the 'correct' form. Seems to me that "Mark is a friend of Bill" sounds perfectly correct, and perfectly equivalent to "Mark is Bill's friend", while "Mark is a friend of Bill's" leaves us hanging semantically close to "Mark is a friend of Bill's mother" without actually going there. In fact, I would propose that "Mark is a friend of Bill's" is ambiguous in a purely grammatical sense. – adj7388 Feb 10 '11 at 22:39
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    @adj7388: "Mark is a friend of Bill's" should be understood as "Mark is one friend out of all of Bill's friends", or just "Mark is one of Bill's friends". – Jon Purdy Feb 10 '11 at 22:52

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