The Double Genitive — English Grammar 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

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):

With Possessive Pronouns

The same pattern works naturally with possessive pronouns, which are already possessive and require no 's:

Common Mistakes

Frequently Asked Questions

When do you add 's and when do you add just an apostrophe (s')?

The rule depends on whether the noun is singular or plural and how the plural is formed. Singular nouns — including those ending in -s — add 's: the boss's office, James's car. Regular plural nouns already ending in -s add only an apostrophe after the -s: the students' essays, my parents' house. Irregular plural nouns that do not end in -s add 's: children's playground, women's changing room, people's opinions. The key question is: what is the base plural form? If it ends in -s, add only an apostrophe. If it doesn't, add 's.

When do you use 's and when do you use an of-phrase?

's is the natural choice for people, animals, organisations, institutions, and time/distance/value expressions: the director's office, the government's decision, a week's holiday. The of-phrase is more natural with inanimate objects and physical parts of things: the roof of the house, the beginning of the film, the colour of the sky. For parts of objects, English also frequently uses compound nouns with no apostrophe: table leg (not table's leg), car door handle (not car's door handle). The rule of thumb: if you can replace it with a person's name, use 's. If the owner is an inanimate object, prefer of or a compound noun.

What is joint possession and how do you show it with apostrophes?

Joint possession means two or more people share a single item. In that case, only the last name takes 's: Tom and Sarah's wedding (one shared wedding). Separate possession means each person has their own. In that case, every name takes 's: Mark's and Lisa's offices (different offices). The grammar follows the logic: if one apostrophe covers both people, it must go on the final name in the pair. If each person owns their own item independently, each name needs its own possessive marker.

What is the double genitive and why is 'a friend of Sarah's' correct?

The double genitive (also called the 'of-possessive') is the construction: a/an + noun + of + possessive. 'A friend of Sarah's' means one of Sarah's friends — the possessive 's is required because the sentence implies Sarah has multiple friends and you are referring to one of them. Without the 's, the meaning changes: 'a friend of Sarah' would suggest the friend is somehow about Sarah or depicts her. The pattern extends to pronouns (a friend of mine, a colleague of yours) and to plural names (a neighbour of the Smiths'). The double genitive is obligatory when the indefinite article a/an precedes the noun.