PERSON + 'S + THING
 
We use this form to express possession or relationship when talking about people.

Phoebe has a cat = This is Phoebe's cat

The leg of the chair
The leg of the sheep
Paul's leg

Spelling:

singular noun My father's car
plural noun My parents' house
singular ending in "s" or similar sound Your boss's office / Alice's book (pronounced: /bɒsɪz/ /ælɪsɪz/)
plural not ending in "s" The children's room

The "Person" may be a phrase:

That is the man next door's wife  (Person= the man next door)
Paul and Mary's dog
Henry the Eighth's six wives

Notice:
Paul and Mary's dog = Paul and Mary have a dog, his name is Tobby
Paul's and Mary's dogs = Paul has a dog called Tobby, and Mary has a dog called Dodo

 
    But if the "Person" is a long phrase or sentence, we prefer the construction with OF

         This is the office of Mary and her new boyfriend
         That's the house of the man who was talking to you at the party

    A Saxon genitive can also be the "Person" of another Saxon genitive

        This is Monica's son's toy    (the toy of Monica's son)
        My brother's wife's name is Karen

Careful with the article! 

We only use the article with the Person if the Person has an article:

That's THE BOSS That's THE BOSS's car
That's PETER  That's PETER's car  (not: That's the Peter's car)

You can read more about the Saxon Genitive here: Saxon Genitive Special Cases.