When the noun is singular, we make the possessive case by adding ’s to the singular.

Examples are given below.

The boy’s father

The girl’s mother

If the word ends in –s, only the apostrophe (’) is usually added.

For goodness’ sake

Moses’s laws

When the noun is plural and ends in –s, the possessive case is formed by adding only an apostrophe.

Examples are:

Boys’ school

Girls’ college

Horses’ tails

When the noun is plural but does not end in –s, the possessive case is formed by adding ’s.

Children’s books

Men’s hostel

When a sign or a title consists of several words, the possessive sign is attached to the last word.

The Sultan of Bijapur’s jewels

When two nouns are closely connected, the possessive is put only to the latter.

William and Mary’s reign

The possessive case is mainly used with the names of living things.

Examples are:

The Prime Minister’s bodyguards

The monkey’s paw

The possessive is also used with the names of personified objects.

Fortune’s favorites

At death’s door

Nature’s laws

The possessive is also with nouns denoting time, space or weight. Examples are:

In a year’s time

A week’s holiday

Five minutes’ walk

A pound’s weight

Some common phrases using the possessives are

At his fingers’ ends

For mercy’s sake

To his heart’s content

A boat’s crew

At his wit’s end

The possessive of a proper noun is often used to denote a building or place of business.

The memorial service was held at St. Paul’s.