The Double Genitive
Use the a … of …'s construction to identify one item from a larger set: a friend of Sarah's, an idea of Tom's, a painting of Picasso's — 10 exercises.
The Double Genitive: A Friend of Sarah's
The double genitive — also called the of-possessive or post-genitive — is the construction a/an + noun + of + possessive. It is obligatory in English when the indefinite article introduces one item from a set of items belonging to or created by someone. Research from the English Profile Programme confirms that the double genitive is acquired reliably only at B2 level, with learners at B1 consistently producing the simpler but semantically different form without the final 's ('a friend of Sarah' instead of 'a friend of Sarah's'). The stakes are real: omitting the 's changes the meaning from ownership to depiction or topic.
The Meaning Contrast
a friend of Sarah = a friend who depicts Sarah or is about her — different meaning
The Pattern
The structure requires an indefinite article before the noun, of to introduce the possessor, and 's on the possessor noun (or a possessive pronoun):
a neighbour of the Smiths' (= one of the Smiths' neighbours)
a suggestion of our manager's (= one of our manager's suggestions)
a painting of Picasso's (= a painting Picasso created — not a portrait of him)
With Possessive Pronouns
The same pattern works naturally with possessive pronouns, which are already possessive and require no 's:
Common Mistakes
✗ Is that a painting of Picasso? (= a painting showing Picasso) → ✓ A painting of Picasso's. (= by Picasso)
✗ A neighbour of the Smiths complained. → ✓ A neighbour of the Smiths' complained.