The most common way of making genitive forms in English is by adding the possessive ‘s ending to the noun, like for example:
Jane’s cat
John’s books
or just the apostrophe in plural if there is an s at the end:
boys’ room
the kids’ toys
In all the examples above the genitive suffix is attached to the head of the noun phrase.
What About Genitive Forms of Longer Noun Phrases?
If there is a longer noun phrase, the genitive ending is added to the last word in the phrase, which in most cases isn’t its head, for example:
the man next door’s house
the woman with the red hair’s dress
These are the so-called group genitives. They’re especially popular in spoken, less formal language. Let’s have a look at some more examples:
my father-in-law’s boat
the mother of three’s cousin
the owner of the factory’s salary
someone else’s problem
Henry the Eighth’s wives
the teacher of geography’s desk
Coordinate Constructions
Group genitives are often used with coordinate constructions, so constructions with a linking word like and or or. Some examples:
Mike and Steve’s room
a minute or two’s rest
As far as coordinate constructions are concerned, you must be careful, because besides group genitives there are coordinated genitives. Compare these two phrases:
(A) Ben and Sarah’s books
(B) Ben’s and Sarah’s books
In A there’s a group genitive because Ben and Sarah are one entity, the books belong to them both. In B, on the other hand, Ben and Sarah are two separate entities and we’re talking about Ben’s books and Sarah’s books, which are not the same thing.
Another example:
(C) Smith and Wilson’s theories
(D) Smith’s and Wilson’s theories
Again, in C these are their common theories. Both Smith and Wilson are their coauthors. So, this is a group genitive.
In D, however, there are Smith’s theories and Wilson’s theories, each set distinct from the other, so this is not a group genitive, but rather coordinated genitives.
In A there is joint possession of the books. In C there is joint authorship of the theories. These are common situations when we use group genitives with coordinated constructions. Other typical examples are joint responsibility (E) and joint relationship (F):
(E) Henry and Mary’s reign
(F) Luke, Steve and Monica’s parents
In more formal language very long group genitives are frowned upon. Instead, of is usually used like for example:
the speech of the king of France