To make singular nouns possessive, add an apostrophe and an s. For plural nouns (except for a few irregular plurals), add an apostrophe only.
- the horse's mouth
- the puppies' tails
- the children's desk
There are, of course, exceptions. In the following notable case, tradition and euphony dictate the use of the apostrophe only:
- for appearance' (conscience', righteousness', etc.) sake
In another instance, the possessive singular of such uninflected nouns as series and species is also formed with the apostrophe only, although the more usual way to express possession with such nouns is by the prepositional phrase: of the species.
Closely linked nouns are often considered a single unit in forming the possessive, when the entity possessed is the same for both:
- my aunt and uncle's house
- the skull and crossbones' symbolic meaning
When the "ownership" is separate, however, both nouns take the possessive form:
- our son's and daughter's friends
Analogous to possessives, and formed like them, are expressions based on the old genitive case:
- an hour's delay
- in three days' time
- Charles's having been there
In the last example, the genitive Charles's "possesses" the gerund having. Should the following word be a participle, or gerundive, however -- that is, a verb form used as an adjective -- the case is not the genitive and the apostrophe and s are not used:
- Peter was annoyed by Carrie's reading the letter.
- Max was delighted to see Carrie reading the letter.
Source: The Chicago Manual of Style