THE POSSESSIVE FORM OF NOUNS

The possessive form is used with nouns referring to people, groups of people, countries, and animals. 'Belonging to' or 'ownership' is one of the relationships it expresses :

John owns a car. ('John' is the possessor or owner)
It is John's car.

America has some gold reserves. ('America' is the owner)
They are America's gold reserves.

It can also express other relationships, for example:

  1. where someone works or studies or spends time:
    John goes to this school. This is John's school.
    John sleeps in this room. This is John's room.

  2. a family relationship:
    John's mother
    The Queen's daughter

  3. qualities:
    John's patience.
    The politician's hypocrisy.

Form
To form the possessive, add 's ('apostrophe -s') to the noun.
If the noun is plural, or already ends in -s, just add:' (an apostrophe).

For names ending in -s:
In speaking we add the sound / z/ to the name, but in writing it is possible to use either 's or just '. The 's form is more common. e.g. Thomas's book, James's shop.

Examples:

The car of John = John's car.
The room of the girls = The girls' room.
Clothes for men = Men's jobs.
The sister of Charles = Charles' sister.
The boat of the sailors = The sailors' boat.

There are also some fixed expressions where the possessive form is used :

Time expressions Other expressions
a day's work For God's sake!
a fortnight's holiday a pound's worth of apples.
a month's pay the water's edge
today's newspaper a stone's throw away (= very near)
in a year's time at death's door (= very ill)
  in my mind's eye (= in my imagination)

The possessive is also used to refer to shops, restaurants, churches and colleges, using the name or job title of the owner. Examples:

the grocer's, the doctor's, the vet's,
the newsagent's, the chemist's, Smith's,
the dentist's, Tommy Tucker's, Luigi's,
Saint Mary's, Saint James's.

a. Shall we go to Luigi's for lunch?
b. I've got an appointment at the dentist's at eleven o'clock.
c. Is Saint Mary's an all-girls school?